from what I have seen in my area (Kansas City) you can get that year boxster a little cheaper than 25K. For around 27K you can get a 98 (I think) Boxster S with around 37K-40K miles. I am looking hard at S models until I find one perfect for me once I start working again. Don't laugh at me for saying this, but if you have a CarMax around, I would go there. I know many people that have went there and it was very easy to deal with, more than accomodating, and they know how to service all thier cars. Even the oil changes in the Boxster are cheaper there. Nicest thing is, if you drive a boxster or an S and want a different color or whatever, if that lot doesn't have it you can have one shipped from another dealer. Shipping is 350-500 dollars depending on from where to where. If you do buy the car the money you paid for shipping goes towards your deposit.
I read that you mention the Tiptronic (whatever it is), I have read lots of people having issues with it starting to slip. I would go for the manual. Of course I am one of the people that think if you get a sports car it HAS to be a stick.
I think you asked about insurance....not sure what to tell you. Different areas have different rates. I think full coverage premium policy for me through state farm on a 2000 Boxster S was 105 dollars a month. But keep in mind I have been with them for 11 years, 9 cars, 4 speeding tickets, one accident (not my fault), and a sportbike. You will pay a high price for anything newer if you are not over 24.
My best advice is......
Seriously check out CarMax. I have heard nothing but good things from friends, and plan to use them myself when I get a Boxster.
First most, don't settle. I may have to wait longer until I can afford it, but I will wait until I can get an S. Since you are younger maybe that is not your best option, so the base may be better. Since it will be a good sum of money I won't settle for anything less than perfect for me. I would rather pay 1 or 2 grand more for the combo I want (either dark blue, grey int. with full silver trim OR deep yellow with black int. with full silver trim) rather than what I don't really want ( ex. the more pastel toned yellow). About the only thing I would take over it is a vette. If you are going to pay for it, then only pay for the one you really want.
MY personal opinion is DO NOT get a Z3. I just think they are too girlie of a car. Although females do look hot in them!!! My ex actually has one and WOW. She looks hot as hell in it but it just seems so slow. It is a little quick, and handles ok....but people call the boxsters cream of the crop for a reason. I can't wait for the day to pull up in an S and rip her a new one...again. Besides, her Z3 is that pastel yellow
Good luck searching, and I know how it is to have new credit. Just don't jump into anything.