You're welcome :-)
In theory you can use multiple print heads for multiple colors, or even two print heads to simultaneously print a second object "clone".
Every printhead needs to be adjusted and calibrated so your different print layers will align...
3 Printheads are a nice idea, but except the price issue it also means a bit more tinkering.
From Model to Print / Setup:
Once you have managed to get your printer calibrated well it does not require a lot of tinkering anymore to get started with printing. But there is always a chance it will be necessary.
It would be advisable to go with a printer design that has been around for some time to avoid surprises and have reliable mechanics.
Printers like the UP will work out of the box and have a limited number of presets, they will give you good results fast.
But sometimes on rare occasions, you wish for more flexibility in the settings.... But believe me, with Slic3r or other open source software and printers, you wish often for a setting that will work after printing three failures... :-)
Resolution VS software:
Thiner layers and finer nozzles will get you so far, but it's not always the printer's hardware that gives you the best quality. The software is very important as well.
If you check out the Make-Article, you will notice a lot of differences.
Makerbot Replicator 2
Afina/pp3dp UP from $899 (Mini)
(this is made with the bigger UP, but still with the old firmware and 0.2mm layers, 0.15 is available now for the big one and 0.2mm for the mini)
Printrbot from $399
(IMHO this can be a lot better, probably the replicator printer as well)
Only the UP was capable of printing the fine details of the hole in a owl model's beak http://makezineblog.files.wordpress.com/.../ow_tn.jpg
The good news is that the Reprap-based printers can be fine-tuned both hard- and software-wise to produce high quality results. It really just depends on how much time you invest on getting to know your printer and software such as Slic3r... LOTS of settings to learn about ;-)
About moving parts:
I make a lot of moving stuff, it is fun to do. But I only have printers with a single printhead. The UP does a amazing job at creating break-away support, so I was able to print gears, ball-bearings and more in once piece and they where movable after cleaning them with tweezers. I also make a lot of tiny parts ( http://thingiverse.com/Schorhr
a) it is possible to create moving parts with just one extruder
b) sometimes it is easier to print the parts and assemble it later. No cleaning/dissolving, no printing tollerances/fusing to consider... Saves a lot of time, and super glue works great.
Plastic colors are nice, but coloring them with model paint, nail polish or such looks much, much better.
But yes, there are many colors available; both ABS and PLA. Even gold/silver/copper, but they don't look as nice as painted. Neon-Colors look awesome though, but still like plastic... If you print something like a model car, painting it will look MUCH better.
Paper, Mcor for example, are now in many Staples as far as I read.
The type of printer has it's charm, but as some plastic-layer printers there's a disadvantage: It prints a stack of paper/plastic and cuts & glues/laminates them together layer by layer.
If you print a single, small object only, the rest of the material is waste. So you have to fill your print area to save money.
To some extend this applies to SLS/Powder printers as well (as not all excess material is recyclable.
Support-Structures are not as much of an issue, but cleaning can be (AFAIK).
Neat stuff. Some printers can do full color. The downside is that there needs to be thicker walls, more clearance for moving part.
They get hardened with a resin, but they are brittle on the inside.
-Nylon/Plastic powder, SLS:
Neat stuff. High detail, flexible parts, rigid... And the powder acts as support, so you don't have to worry about overhangs and moving parts as much as with plastic extrusion. Downsides are the limited reuse of excess material, the long heat-up and cool-down phases, and it can be a mess... powder! :-)
After the powder chamber get's heated somewhat close to the melting point, a laser fuses material. It's fast once it's running...
Warping can be an issue but when a machine is set up it's awesome.
Machines are expensive, and commercial big machines usually have high material costs and pricey service plans, but of course it depends on the machine.
Powder is similar to the Nylon SLS; Metal powder get's fused by laser, and afterwards gets "baked"/Infused. Gold, Silver coatings etc., very nice for jewelery, not sure about wearing stuff though (I once read that someone's printed ring made dark spots on the skin).
I also heard that it can be dangerous if the fine metal powder drops into your shop as breathing it in is verrry unhealthy...
Liquid resin that hardens under visible- laser or UV light- High detail prints, transparent materials available... I want one of those :-)
I have not seen one myself other then on internet sites, but I would imagine it can be a bit messy when something goes wrong :-) Resin isn't exactly cheap either.
Cheap machines, cheap materials, simple. That's why it is so popular... Repairs for Reprap-based devices usualy cost little (integrated board can cost as low as under $100, motors $10-20, Structure parts can be printed or bought in a hardware store... Great.)
Is it possible to use PLA and ABS in same printer?
Yes. If the printer has a extruder that can go hot enough and a heated bed it is suitable for ABS.
For PLA fan-cooling is required, as it gives better results when the plastic gets solid right away.
Transparent plastic is available, but it's more translucent (you can try trating it afterwards though). Real glass can be used in some powder printers, but it's a bit Milky white.
You could allways use a plastic printer, create a casting form, and make a resin part out of clear resin. Or a resin printer... :-)
1kg/2.2lbs of plastic usualy cost €18/$23 or more, probably cheaper if you get lucky or order large ammounts. Quality does vary a lot, and different plastics and colors have different melting temperatures, properties and such.
One reason the UP is so good is because the OEM plastic is of constant quality, but costs about three times as much as cheap plastic from Reprapworld.com for example..
When you allways shop for the cheapest filament you will need to change your printer software settings. Even if you use the same stuff all the time, it can happen that a new shipment is a bit different... so it's not allways good to get the cheapest available. Also some cheaper stuff may be smaller or thicker then claimed.
If you have a extruder for 3mm plastic and the plastic is 3.2mm thick, it can cause jams and you will need to take appart the extruder and clean it... No fun!
RESIN is allways more expensive. But the different companies and projects are sourcing stuff that is way more cheaper. IIRC commercial resin could cot up to $300 a liter... Some stuff around now costs less then half.
Also keep in mind that on powder or liquid printers, that trapped material is lost unless you create small holes on your model.
Plastic extruders will create a fill-pattern to make models stable but save material, no worry about trapped material.
I don't know of any cheap ceramic or glass printers for home use. The way to go at the moment is to either prototype it in plastic and then get a company to print the model in a material of your choice once you're happy with the prototype... Or use your model to create a cast. It has been done, especially with resin and metal.
I hope this helps. Keep in mind that I'm just a hobby enthusiast and some info might be outdated, wrong or incomplete. Plus it's pretty late here, so I might have a few mistakes in my post even though I should know better ;-)