Lets go to the child seat question first and get the deal breaker out of the way. There is a bar under the seat that is an extra cost option that enables the seatbelt off when a properly equipped child seat is installed. Talk to the dealer and remind him that Porsche typically pays for the installation.
Costs...$50 parts and $30 labor at my nearest quick oil place for an oil change...I bring my choice of oil, filter, crush ring. They do 6 a month. No issues. I watch like a hawk. Could you do it yourself...sure with a big enough catch pan and a wrench that fits the filter.
Cost..I'd do a 90k on a car this old immediately as it is the most comprehensive. Probably $1k-$1.5k depending where you live and who does it...dealer or independent Porsche specialist.
Do get a Pre-Purchase Inspection...
More about buying a Boxster here
https://sites.google.com/site/mikefocke2/mikesporscheboxsterwebpages