How about this. Buy 3 to 4 thous. dollar 1990 miata (maybe even get one with hardtop !!). Use it for autocross for 6 mos to a year while replacing worn parts and getting it into excellent running condition. Then slowly getting it road race legal (cage, kill switch, etc.) and maybe finding a spare set of wheels and so on, but keeping costs to a minimum (read: not going to be competitive for first year of road racing). In six mos to a year I would have a great running car that I am very familiar with (autocross) and the cost was spread over a period of time.
Now here is the question. Do you have to have the spec miata suspension installed before you can run it or just the necessary safety equipment ?