One of the big issues I've experienced with classic cars is the simple weather-tightness. 50+ year old seals have long dried out and replacements just don't fit as well as the originals. I have about $1000 worth of new rubber in my hardtop and I still keep a towel in the trunk in case I get stuck in a hard rain.
With regards to anti-theft, yeah that's a problem too. I installed a simple interrupt device on the hood latch so it can't be opened unless the cable pull in the cabin is in the right position. I also have a GPS locator that uses cell phone service to notify me if the car goes out of a set range. Both these devices can be easily disabled by a guy with a tow truck and an enclosed metal trailer. IMO a classic car thief is going to do that, and the car would be in a container on its way to Eastern Europe or Australia...
The best bang for the buck for the OP author is to buy an S197 Mustang and use that as his DD. They are cheap now, retro styling, with plenty of inexpensive replacement parts, either OEM or aftermarket improvements. Not to mention air bags and a five star crash rating. I bought a well-used 2005 back in 2010 for my daughter when she turned 16 and she still has it. V6 with a 5-speed manual. And we've done some nice mods on it: true dual exhaust (sounds wicked), GT suspension, wheels, tires (handles wicked), HID headlights, and most recently a custom re-paint. It has well over $150k miles on it and runs perfectly.
2005 4.0 MT dual exhaust GT mufflers GT suspension and wheels