I ran the 24 hours of Nelson Ledges in a Saturn SL. It wasn't super quick (obviously), but it could easily make it the whole race on one set of tires and brakes and was easy on fuel. We were able to keep it going without major incident the whole race and we placed 8th our of ~80-90 cars, IIRC. It was a ton of fun as an entry-level car.
I would definitely recommend the Saturn based on that experience. I think it will be far cheaper on consumables and probably more reliable. Parts are also likely cheaper and easier to get when something breaks in the middle of a race. If you're looking for max fun for the money, that's what I'd do.
On the topic of other advice, I'd say it's a really good idea to volunteer at a Lemons or ChumpCar race before driving in one. It'll give you a much better idea of what it takes to have a successful event and you can figure out how tech inspection works, talk to other teams, etc, without the pressure of racing.
Also, I'd really recommend planning out your whole build. Especially the finances and who pays for what, etc. I think it would be very difficult to prep a car for less than $4-5k, and many builds are near $10k if you're buying all new safety equipment, tires, brakes, paying for a roll cage, etc. Be aware that every event could easily cost $2000 or more if you need to tow any distance (race car fuel, truck fuel, tires, brakes, entry fees, etc). Also, know that your safety equipment won't be cheap. If you need to buy everything and want to get it new, you could be spending $500+ easily, much more if you want really good equipment and a Hans device or similar.
As a point of reference, I've been building my own ChumpCar recently and I have estimate that it will cost me ~$4000 to build the car, and that's with me building the cage entirely by myself, mostly using safety equipment I already own (harnesses, net, seat, etc) and generally doing it on a shoe-string budget. Our first event will be a 14 hour enduro and I am expecting the total cost for the event to be ~$2500. So yeah, while it's cheaper than most road racing, it's still pretty expensive compared to most fun weekend hobbies.
Anyway, have fun and keep us posted on the progress!