Not Great, but not Bad either...
Amrossi,
When I had my '72, std 350 engine, IF....... and I do mean IF, I was real gentle; which of course is darn near impossible, I could eek out 17-18 hwy out of her. Mostly 10-12 in town like Jason indicated. This was on the original engine with 175K miles too.
As for reliablility, that gets a bit tougher. A well maintained Corvette should be a reliable caras a daily driver. You'll find many of the folks here on the CAC use their Vette's as daily drivers with no problems at all. Best chance is to find one where the history is documented, even still there's no guarantees.
As for a budget... expect to put some $$$ into the car. Heck, any previously owned car will require some periodic work. Buy a Corvette because you WANT one. There's an enormous amount of expertise in any year of Corvette on the CAC and a ton of folks ready and willing to help you with any questions you may have.
Regards.......... Nut