well, coming from experience, pay the extra $$$ and go for oem. i bought a new aftermarket one off ebay w/ 3 yr warranty and the Crank Poistion Sensor inside was a bit out of spec so the car ran very wierd and gave me a CEL every time i drove.
ive tried to figure it out with them and havent gotten any responce back for an exchange or refund so...

i went with oem and the things you have to buy are the Housing, ignitor, igntion coil, cap and rotor... (and extra bolts/screws if you arent going to salvage them off the old distributor)
you CAN go with an aftermarket ignitor if you want because i went with one and has worked fine so far. not so sure about the ignition coil though.

installation is very easy but make sure you mark the location of where the distributor was set so that it doesnt mess up the timing too much (id still suggest to check timing)