With a C3 Corvette, I think it really depends on what you want to do with it. The more rare 427 engine'd cars can be quite collectible, though prices have bounced around. The unicorn L88 of course will fetch enormous money, but the easier to find L89 will still set you back at least $100K or so, but even the L89 cars have gone up and down $50K or more over the last few years. They're not a "gimme" investment. Buying a $40K L36 might seem like a good alternative but really, it's a hard market to try to predict. I'd only expect to not lose money on one, and if I made some, then bonus. BUT, you have to keep it, and keep it original and not add a lot of mileage.
I'd rather buy a small block car that I could build myself and have a ton of fun with. Either swap a big block into it, build out a screamer small block, or maybe a slight resto-mod approach with a modern crate engine. They're not big cars, but the engine bay is actually pretty accepting of a lot of cool engines.
The C3's IRS is okay, but hot rodders stayed away from it for a long time as you were just stuck with it. There's now some sweet modding support that solves the "not really completely IRS" and camber issues of the Corvette's rear suspension. These can actually be turned into fine handling cars for not a ton of money.
There's kits to easily fit a Tremec if you're looking to move away from the old four speeds. The old Muncie M-22 can hold a lot of power, but if you're buying one and making it new, you're spending a lot of money for a four speed. Expect to drop at least $2K just for a trans and then start rebuilding it, and if someone is selling a set-up and fresh M22 for $2K, they're lying. It won't be hard to end up into the trans for $4K. For $5500 or so you can buy a fresh T600 with all the hardware to bolt into a C3.
I figure if I found a $20K great shape small block car (not numbers matching, etc) with a stick, sold off the driveline, and then started from scratch, I could build a seriously fast C3 for $35K.