I bought my '99 last June for $6800 in California with 50,000 miles on it. Mechanically, I immediately needed to put in a new clutch and rear axle rebuild, adding about $3000 to the investment. Then there are non-critical elements that needed attention. For instance, the stereo head unit was faulty, so there was another $1000 expense (but also a significant upgrade with double DIN, backup camera, touch screen, Sirius, etc.).
My experience seems reasonable and probably typical for a car that is old enough to drink.
And I still have issues to address. Paint restore. Need the steering wheel re-wrapped due to sun damage. Driver's seat is worn and could use a re-dye. Would like more aggressive rims. Those kinds of things. Nothing that is really necessary, just the kind of projects a 21-year old car could use to make it gleam.
All-in-all, I've could easily put more into it than the purchase price. That's the nature of old cars. No matter how well treated, rubber parts just degrade over time. Sensors corrode. Issues crop up.
But with all that, it was completely worth the price and the after-purchase expense. Mrs. Peel is a sweet, sweet ride, and drives like she just rolled off the assembly line. I think a solid well-kept '99 is a great purchaser, and has one of the best buy-values around for any sports car.