A C4 will be a tad smaller and lighter than an AOD, if you are strictly making a track car and can be rebuilt with common garage tools. Find a C4 out of a truck if you are going junk yard shopping.
If you choose to go the AOD route with electronic OD and a manual valve body, expect to pay more in the long run, but with the added benefit of being able to cruise the highway at a lower rpm.
My suggestion is to do it right, put a properly built tranny on the CC if you don't have the funds and do it ONCE and make payments to pay it off as quickly as possible.
I can also vouch that even doing it myself, most T5's cost 500+ to repair just in parts as the broken teeth also chew up the counter shaft. You could go the Astro Performance route but then you still have a weaker case, which Astro has some bracing for that as well, but if you have to pay someone to install the kit, it becomes rather pricey really quickly.