Building a home made 3D printer pen (3Doodler)

I am looking to design and build a homemade version of the 3doodler for use at a table at a local science expo. Knowing there is bunch of engineers on these forums, I chose to bring this little project to the Soliforum. So please, if you have any input at all, post it!

I will break down what I am looking for / currently thinking regarding the design of the printer pen.

Extruder:
Considering the fact that I won't have to compensate for variable speed, temp, and layer height, I don't think the use of a stepper motor is required. What hi-torque / low rpm motors do you recommend? As for the actual extruder body, I was planning on using a reprap design.

Nozzle:
Should I just drill a .5mm hole out a bolt and go from there or should I go the machined nozzle and heater block route?

Hot End:
I do think a variable and monitored hot end with a thermistor is required. This will allow me to switch between PLA and ABS. As for monitoring and adjusting the temp, how should I interface that with a computer? Or should I go the soldering iron approach (no thermistor, just a heating device that will stop at a certain point)? I am open to any and all suggestions regarding the hot end. This will be the most difficult part.

Hot End to Cold End Insulation:
Fiberglass wool? Bamboo? Peek? Not sure what the best material to use is. Any ideas for the transition between the extruder and the hot end w/ insulation?

Plastic Filament:
I am currently leaning toward ABS as it has a smaller cooling window and is a bit less brittle than PLA. However, I cannot chose between 1.75 and 3mm filament. 1.75mm is easier to procure for me but 3mm may be more feasible.

I apologize for all of the open-ended questions. I am still in the feasibility check phase and have not yet begun the design process. I thought I'd bring it to the creative Soliforum for ideas before I began. If you think of anything else I did not mention above, please post!