Steve. Your comment about the car running a little rough (due to needing a warm up) sends up a red flag to me. The owner could have spent a few minutes warming it up for you. In the summer, a well tuned engine should run without hesitation after a reasonable warm-up. If you drive it again INSIST on it being warmed up, then drive it for some time in traffic and at speed to make sure it runs good at all speeds and runs cool.
I would not dream of buying that, or any other used VS/JPS Speedster Replica at the price he is asking (22), unless it ran perfectly, had no mechanical issues, and the paint was 99% (meaning maybe a few stone chips up front) The interior should be flawless. And Brakes? You said the brakes felt weak? The brakes should be at a minimum discs up front and should be confidence inspiring. Red Flag!
Many of us have been "around cars" most of our lives, like you. But, a VW based Speedster Replica is a whole new ballgame unless you have some recent High-Performance Air-Cooled VW time under your belt. If you buy a less than satisfactory Speedster Replica at 18-21G and it's not 100%, you may be in for some rude surprises. Based on your comments it could have burned valves (or worse), cooling problems, Carb issues, brake issues, etc. On the low end the repairs might be $100 DIY stuff, or it might require some major repairs up to 1 or $2,000, maybe even more.
Get a local High Performance VW shop to check it out for you. There are members near you (look in the registry). Write them a PM and ask for advice on shops, what to watch for, etc. The car may need a little TLC, or it may be a dud. Have an expert find out for you.
There are plenty of good VS's on the market for 15-20G. Do not get in a hurry. Bottom line? With the money you are willing to spend, insist on the best and do not even dream of buying a car that has some issues up front.