If you're printing PLA, you are correct, you don't need a heated bed. PLA sticks very well to Kapton tape on an insulating support (i.e. wood -- I have a laminated chipboard wardrobe shelf from Ikea with the Kapton on top). PLA may even stick to Kapton on more thermally conductive supports -- glass, aluminum -- but I haven't tried that. All you need is temperature control for the hot end. As long as you don't want to go fancy and vary the temperature during the print, you can use pretty much any microcontroller with an A/D converter, plus a MOSFET and a few resistors, to build a smart thermostat.
I wanted to limit software DIY to a minimum (I'm a programmer by day, so I know it tends to eat away time like nothing else), so I used an Arduino Mega 2560 with the Marlin firmware, four Pololu driver carrier boards and two MOSFETs on breadboards (one for the hot end, one for the fan). I followed the RAMPS schematic, but at 24 V supply (except for the fan). I have moved the electronics to perf boards in the meantime, and changed to separate drivers for the two Y motors, but still no purpose-made PCB yet. That setup works like a charm*, with stock software: Marlin firmware, Pronterface host, Skeinforge slicer; I use OpenSCAD for design, but I'm not entirely happy with it.
My hot end is a J-Head mk V-B with the 5.6 ohm heater resistor replaced with a 25 ohm resistor of the same type (to work at 24 V instead of 12 V); Digikey has the resistors. The extruder is a Greg's Wade Reloaded (this one
) bought on eBay. That combination worked perfectly from the first try, with PLA at 185°C. I've had no problems with it at all, so I wholeheartedly recommend it.
* I really need to find a better simile. Charms don't actually work at all; my setup does.