Okay your not getting any love here so I will throw my opinion out there. I don't race so my opinion isn't based on first hand info.
I would buy a something like a 2005 or newer 250 four stroke mx bike to start with. To get your feet wet throw on a set of tires on mx wheels, it is refered to as a sportsman set-up and the cost is very low. Older bikes are going to be tough to source parts for and the older stuff you mention is going to be a dog or a 2 stroke. You want a hi performance 4 stroke for sure.
If you like it then you can dive in head first and do suspension, brakes 17' wheels etc. Lots and lots of money. If you can find a bike already set-up then do that.
I think it is 408 racing that does the supermoto races at shanonville and the beginner class is pretty much run whatever you have and also look at esmra series in the states. I think ESMRA runs in NY and PA.
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