As for the top, I would not put the real expensive one on because of the age of your Boxster. You can get them from $350 to $500, just check around and check out Mike Focke site. Great info here.
http://sites.google.com/site/mikefocke2/mikesporscheboxsterwebpages
Interior, check the classifieds here and Pelican and other Boxster sites. There are guys parting out cars or converting them to track. Great buys can be had.
Paint, that's about the goin rate.