I've had a similar idea for a while now but after happening upon this thread on CPF I'm starting to lean towards something simpler.
All the cheaper 18V drivers are crap (exhibit A; exhibit B; exhibit C) and a decent one costs half the price of a ready-made drop-in.
Doing the heatsinking is going to be tough as it is without having to build the reflector and driver/led board from scratch.
[EDIT] Alright, this driver seems okay.