Jim's not here right now ....
Tommy, I'm sure Jim will reply (we know each other well), but I'll give you my 2 cents worth.
If this is your first race car, buy - don't build.
Unless you have mad fabrication skills you want to show, and sell, making a street car as modern as the E36 into a race car can be soul (and wallet) destroyer. Jeez, the wiring harness alone weighs about 100lbs, and knowing what to cut and what to take out can take hours.
The E36 is a pretty safe bet for BMW CCA club racing, NABR (with NASA), and SCCA IT. Read the rule books cafefully, and know what each allows, and doesn't allow to the class you intend to run.
A stock M50/S50, or with the usual bolt-on mods (assumming the class rules allow them), is pretty bullet-proof. Pay attention to the usual weak points; Cooling (plastic radiator neck), oil-pump nut, timing chian tensioner, motor mounts. Change the fluids regularly, like 3 times more often than a street car.
The chassis has a few weak spots; rear sub-frame (particularly on the non-M chassis), rear sway bar mounts, front sub-frame at the motor mounts (rarely), and the lower trailing arm mount/toe plate attachments. A good rollcage makes for a good chassis :)
Ask lots of questions, watch local races with E36's, check out cars for sale.
Some examples (they may be sold or not, I don't know any of them)
Feel free to ask me more. I the crew chief for a 5 car syndicate (with an ex-Group N E36 M3, 2 ex-Speed Touring E36 328's, a very quick E30 M3, and a work-in-progress E36 M3)
regarding "buying and not building"
...I'd have to agree.
It'll take a very long time to build a well sorted-out car. At the end of it all you would never be able to sell it for what you think you might be able to. There are many E36 325i Jstock, JPrepared, and SCCA IT cars out there. Another gentleman that you might want to look up is Xavier McClure. Very nice guy, he has been racing for YEARS and recently moved up into the realm of the E36 chassis. He's extremely mechanically-ept but decided to just buy a car that was already built. At that point you might only have to redo seat mounts, replace harnesses, brake job, maintenance, etc.
Just my two cents.