I'd guess this is probably the item you need:
http://www.pelicanparts.com/cgi-bin/smart/more_info.cgi?pn=8N1-837-015-C-M100&catalog_description=Door%20Lock%20Mechanism%20Left %2C%20Boxster%2FBoxster%20S%20%281998-04%29%2C%20Each
So not too bad. I haven't taken my doors apart ever, although it's on on my list of things to do. I have a bad window motor on the passenger side that needs sorting out. I'd hope that replacing the whole assembly is worst case. It could be frayed wires shorting together or getting pinched by the window or such. I'd imagine the lock (tumblers part) will be transferrable from the old assembly to the new assembly, but that'd be a good thing to check before you do too much.
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