What you are thinking of doing is exactly what I did.
I bought a bone stock 986 base and did 15 track days learning to drive before I improved anything (street tires and brakes) on the car.
Then I upgraded the seats with harnesses and changed to sticky street tires and street/race brake pads and did another 10 track days learning to drive better still. (
Street or R-Compound Tires?)
Only when I was finally good/fast enough to post lap times at the top of the stock street class did I modify the car and moved from street class to BSX by adding the full Spec Boxster suspension (PSS9's, GT-3 sways, etc). (
http://986forum.com/forums/general-discussions/35845-suspension-upgrade-week.html)
Then I drove in another 25 track days in BSX to work on getting fast. Eventually, I won 14 time POC trial events and set POC two track records in BSX.
Then I built a Spec Boxster, passed the racing clinics, and went wheel to wheel racing. (
Spec Boxster Build).
It has taken me four years and 95 track days to get to where I can now run in the Top 5 in Spec Boxster racing. Of course, others have done it in less time and less money. This is simply what it took me to get to this level. Your mileage may vary.
Its a long road and a serious commitment to get to the racing level but
worth every minute and every penny. Nothing can compare to the experience except maybe getting married or having children.
Very few can say with confidence that they are a legitimate race car driver. It is a major life accomplishment and something that no one can ever take from you. You will learn more about yourself through this process than about high performance driving or racing.
Some people think that driving a car fast on a racetrack is easy - get in, floor the throttle and turn. We all drive everyday, how hard can it really be?
The reality is that racing is life. Its every emotion, every challenge, every battle, every joy, every burden, every fear, and every blessing. Its raw, its passionate, its painful, its humbling, its freeing, its dizzying, and its ecstasy.
Its all of your years of living wrapped into 1 minute and 33 seconds. Its like taking a cup of life, pouring it over your head, and letting it wash through your mind and body.
To answer your question, I would recommend buying a base 2.5L 97-99 to start. You'll need to get used to the (relatively speaking) lack of power and work on maintaining momentum. We have all seen drivers who rely on power to make up for poor driving technique - don't be that guy. Once you can drive a (relatively) low powered car fast, then you can drive a powerful car even faster.
Of course, I am a firm believer that there is usually many paths to reach the same destination so feel free to pursue whatever path you feel is best for you.