What must you fix?

Here in NC, it boils down to safety and emissions. Safety you absolutely want to do right. Safety in this context is brakes, tires, steering, suspension and lights that meet the OEM standards. Not a high bar to clear, and will actually lead to more fun as the vehicle is more reliable. Emissions in NC are on a 20-year rolling cycle. If the model year is 20 years old or older, it's not required to pass an emissions test. This alone can save you thousands of dollars if the catalytic converters are gone. Whether you want to contribute to NOx emissions in your neighborhood is up to you at this point.

Optional fixes for money saving and more fun

Youtubers can show you how to perform almost any repair. Only experience will allow you to close the gap towards professional quality. Decide now what level of panel gap tolerances, paint imperfections, and fit misalignments you're ok with. This is where the vast majority of your costs savings will be compared to full restoration by a professional. For example, the author bought a 2013 BMW X3 with a front right fender that had been ripped off. I simply bought a used fender from ebay that was the same color and installed it. The hood did not fit precisely, it needed to be coaxed into place with some amateur-ish bending. The critical eye would see that it did not fit according to the repair standards. This gap of 'good enough' that I decided not to fix is what saved me $1-$3k.

Are there other features you can 'fix' by pulling a fuse? On the BMW 325Ci, I decided to buy an emergency car cover, and bolt the convertible top in the open position for a few months of fun summer driving. Fixing the top hydraulics would have cost much more than the cost of the car. The cover cost $100.

The 'fun' context for me is daily driven on public roads. This means that safety and emissions inspections are required. That's a low bar, and clearly defines which parts must be purchased.

On the BMW 325ci, the dashboard was dead. Ever drive a car with no speedometer, tachometer of indicator lights? It's actually fun and not illegal. All of the other safety related systems are still fully legal and pass inspection. If you really want a speedometer just fire up any number of GPS apps on your phone and you're good to go. Save yourself a few hundred dollars of replacement on a 200k mile 20+ year old car. Fun.