Costs are variable, depends a lot on the car you buy. If you're lucky, you only will need regular maintenance, tires, and brakes. Not lucky, you'll need an RMS, your window motors will fail, O2's will go, MAF, etc. If you do your own work, and you're good, you'll save over 50% of the cost of repairs by avoiding paying for labor, assuming you already have the tools ($$) and access to facilities ($$).
In my experience, most people generally under-estimate the true cost of ownership of any car. My suggestion is that unless you can
afford regular maintenance and the cost of likely repair (and on Boxsters, this has to include the possiblity of engine replacement), then buy a cheaper to own and maintain vehicle, and wait until you can afford what you really want.
RE: aftermarket warranties - I have one, but if you read postings here, getting them to pay for a major repair can be difficult. Caveat emptor.