How much upkeep do classic muscle cars require?

I have loved classic American muscle cars all my life and have finally got to a stage where I can afford one.

But I don't want to invest if I won't have the time to keep it in top working condition.

How much maintenance work/costs does it roughly require to keep cars like a 1969 Chevrolet Camaro or 1967 Ford Mustang in good working order?

If its an investment, don't even bother. It takes a weekly wash like a new car, and replacing broken parts like a used car. If you don't drive the hell out of them, just routine maintenance.
that is one of the joys of owning a classic, spending that time on them. I can change plugs in less than 5 minutes but generally milk it out.
don't fool yourself, it takes money, work, effort to locate parts, and the ability to modify or change something if the parts arent available( mustang or camaro i doubt youll run into that).
its not about the expense, or originality, but in about keeping it going down the road with the american hot rod spirit. Some people focus on all original, some don't, but the common denominator is the spirit, the nostalgia, etc.

I think, to own these things you should have very good passion and somehow have a good amount of money. And you may talk to some people who own these to get a clean view of your requirements. You can also join some forum or fan club.

A lot, especially in the UK (which is where the question is showing). Also depends on how much work/effort you're prepared to put in yourself.

Not clear if you're investing (150,000 +) or looking for a driver (30,000 up)
insurance cost is high
The only people I know and have met, It is a life style it will consume all your spare time if you choose a driver… And the investors the trailering and hermetically sealed climatecontrolled garages consume dollars.
Check with your financial/tax advisor… Sometimes the hobby tax laws are an advantage… Or forming a LLC can soften the blow…
then maybe you just want a beater for 15 20k plan on another 5-10k in expense annually…