I've got to weigh in on this one as well. I'm w/ Kirk on the buy vs. build issue, especially for your first car. And while I think it's appropriate to ascribe some value to your time, I also can understand the 'fun' aspect of building a car. Not to mention the in-depth knowledge that you'll gain about the car.
However, unless you have a shop to work in, are fairly good at working on cars, and have some specialized tools (mig welder, etc.), I would really suggest that you re-think buying a prepared car. If you want a 2nd gen. RX7, that's fine. I'm not sure what kind of shape your '90 GXL is in from a street car perspective, but have you thought of selling it and using the proceeds towards buying a prepared car?
I think Kirk's right on target when he says that people underestimate what it will take to get their street car on the track. And that can lead to a couple of things happening, neither one of which is very good. On one hand, you may end up spending a LOT more money than you thought you would (or that you would have had to pay for a prepared car). On the other hand, you may not have the extra funds beyond what you've budgeted (and won't go credit-card racing), so that you have to start cutting corners. I'm not saying that you have to go out and buy the best or most expensive bits and pieces for your car, or try to build a totally developed car right away, but if you have to control costs by going w/ less than desireable alternatives, you're probably not going to be happy with the result.
When I first wanted to get into racing, I had a Rabbit GTI that I had been driving on the streed and running Solo I events with. I loved the car, and was very comfortable driving it. I had gotten to be pretty quick in the Solo I stuff as well. So naturally, I wanted to build it into a race car. I didn't intend to build a full-blown car right out of the box, just something that I could get on the track. I was going to go w/ a good racing suspension (but not all the way to coil-overs), have a custom cage installed, buy an extra set of wheels, 2 sets of race tires, and have the car painted. And of course the standard safety stuff (belts, window net, etc.). I was going to run the stock motor and transmission, at least for a little while.
Well, I went to speak w/ a guy I new that did car prep and built cages. We sat down and talked about what my goal was, and what I was planning on doing w/ the car. We came up w/ a list of what needed to be done to the car, and started figuring out how much it would cost. At the time (1995), w/o doing anything to the engine/transmission, it was going to cost me somewhere in the neighborhood of $6000 (and a good 2 months work of work) to get a car that I could take to my first driver's school.
Well, in addition to doing car prep and building cages, this guy also rented race cars, and had a couple of cars for sale. We talked about the whole rental option, and I decided against it for financial reasons. I would rather be in a position that I have to sit out a few races (season?) if my car got wadded up than to have to write a check for several thousand dollars to pay for a rental ride that got wadded up. But that's just me.
Then we started talking about the cars that he had for sale. One was an ITC Toyota Corolla and the other was an ITA MR2. Both were already prepared cars w/ log books. The Corolla was more developed than the MR2, but probably would have required more care and feeding. Bottom line is that I was able to buy the MR2 for a little more than half of what we figured the cost to build my car was. And that included a reasonable spares package (motor/tranny/suspension parts/some body parts/etc.). It got me to the track 2 months earlier, I had a car that was a VERY good car to learn to race in, and I got some track-side support from this guy since he new the car.
I ended up running 4 races w/ the car that year. By the time all was said and done for the season, I spent less money buying the MR2 and running it for the season than the initial build budget for my GTI was.
Anyway, sorry for the long-winded post. Good luck w/ getting a race car, whatever way you go.
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MARRS #25 ITB Rabbit GTI