A parking lot purchase should get a parking lot price. I would not want it to be my first machine, but at this point I can look inside and take a reasonably educated guess about what kind of shape it is in.
-Are boards/major parts missing?
-Burn marks on boards/connectors?
-How clean is it overall/what kind of life has the game had?
-Acid damage to the board(s)
Also what is the game, something dirt common with a bunch of cheap replacement parts everywhere or something that has some very hard/expensive to find parts.
Also, no question, cash in the only way to go. I personally would not leave any cash in my car. A couple of thousand in $100 bills in very small. If the show is in an area where you fear for your personal safety with carrying around some cash, just don't go to that show. I would be far more nervous with cash in my car. My family has had about ~15 car break-ins over my life, not once has someone approach us to attempt to rob any of us personally.
It is not as though a big sign appears above your head "This guy has $10K in cash on him". No one will know you have the cash unless you have a fat stack of bills peaking out of your pocket and unless you are in a strip club trying to get lots of "attention" I would not ever encourage that method of carrying cash.