I found a running 90 miata here in town for $900. If I were to take the $3500 I had set aside for a "prepared car" and buy the miata...could I get it on the track for the other $2400?
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i'm going to make a few assumptions based on this statement which may be wrong. please don't be insulted if they are. just trying to help here.
if you say you have $3500 set aside to buy a 'prepared car' that tells me a few things;
1) if you're looking at an RX-7 that tells me at least ITA, maybe ITS. you're only going to buy a junker or someone elses failed project. bad idea unless you REALLY know what you're doing/looking for.
2) a $3500 purchasing budget tells me you should be in either ITC or ITB. the reason i say this is that WAY too many people end up buying cars they can't afford to race. going faster costs more money to keep the car running properly. you'll spend more in tires, brakes, consumables like wheel bearings, ball joints, etc the farther up the speed pyramid you go. after a couple years of struggling they get frustrated, sell the car at a loss, and leave racing altogether. this is not what we want to see.
3) you really need to be buying a built car and not building one. $3500 will get y ou a decent donor, cage, and wheels/tires. i know you said you enjoy working on stuff, but don't worry, there's ALWAYS work to be done. i haven't had a thing break in almost 2 years on mine and i always have work to do between every event.
i also think it's really important for someones first venture into racing to get a common, well established car. the commonality of parts, known setups, and friends you'll meet at the track that can help you are a HUGE benefit most overlook when looking to get into racing. an ITB CRX i think is probably the best choice for you, not sure about ITC, but my gut tells me an old civic is a good choice. the ITB Porsche 924 may be the coolest choice, but probably not the best option for someone just getting started, or the cheapest.
if you REALLY have to get into ITA, wait until you can save up the cash to buy a nice car to get you going. i built my first one and it took me 2yrs and about $10,000 before it wasn't a huge pile of crap. you can buy a nice ITA car for about $7500, whereas a nice ITB car may only cost you $5000.
good luck.