Chuck, Chuck, chuck... hello, ... it would be nice to do a track day some time soon, it's been a while.
However, It seems to me that the cost of your proposal is not cost effective. You might pay what $20-25 each used tire an $60 mount and balance (x 2 if you gonna replace your street tires on your rims. Or do you have 2 sets of rims? ) = $220. + trusting your car and life to questionable used tires?! could get pretty expensive ...
What's so special about the ones you got on now?
What pads and brake fluid you using? That's the big question.