I have just gone through two tank conversions and have all the do's and don'ts! At some point, I'll do a complete tutorial on the cheapest method possible.

For very quick reference:
1) Start with a HL IR tank as it provides the correct recoil, Applel and barrel options (do NOT buy a HL tank with metal treads - you'll need to start upgrading other components and the plastic treads are lighter and easier on the tank)
2) Buy Darkith's BDC-2 (use a better hobby radio if possible) and DBU kit (they are fairly easy to build if you are careful AND TEST EVERY CONNECTION PRIOR TO HOOKING IT UP!
3) Modify the recoil unit with a microswitch and use a PICO Switch to power the recoil unit from the recoil servo lead on the DBC-2
4) Use the HL Apple and install the LED's from Darkith's DBU in the turret and feed the 3 wires to the HL Apple Sensor.
5) Modify the HL Apple sensor by replacing the HL sensor in the Apple with 1 of the 4 sensor's that came from the DBU kit

And BINGO - go kill some $1,000 Tamiya Tanks for pennies on the dollars.
One of the best HL Tanks to use is the KV-1. It's light, tracks hold up well and has enough room to work in. And it's cheap.

Good luck - it's a blast to kill the big T's with a HL (they get all flustered if the HL doesn't break - when it does, everyone has a good laugh).

However, I do dream of buying a Pershing Tamiya that works right out of the box!