http://www.autoblog.com/2012/12/12/i...ng-rental-car/
As many as you're willing to pay for. There are cars that have been racing since the 50's with owners willing to pay the costs to keep them in good repair. I don't think your targets are unreasonable (especially since you're already budgeting for a full rebuild of the expensive bits) but by the time you get there it could be quite expensive, we have no idea what the next 15 years will bring for the auto industry.
Your plan is ok for now, don't be afraid to adjust when things change, and don't forget that there are many cars in much worse shape still banging out laps:
[ame="http://www.youtube.com/watch?v=Of_nSoDIDgA"]24 Hours of Lemons and the Mercedes C63 AMG Coupe - YouTube[/ame]