Silver shadows! I got one of those I inherited, and if you can understand the braking system on one of those you will find a 996 a walk in the park! Seriously!
I think its a good idea to keep everything that can be stamped in the service book, stamped by a main dealer or in my opinion by an independent who I think will give your car more care and attention.
As for other jobs, the main dealers can have a nasty habbit of doing any jobs they think need doing wihtout asking, however an independent like the one I use may be more helpful with you doing your own jobs if they know you like doing them.
I have a deal going like this with www.jmgarage.com in Bournemouth, they know I like to get my hands dirty, so they go to town on servicing looking for anything to add to a list to hand me to get sorted out, they even sell me the parts to do it and sometimes work with me by being happy to look over what I have done when I pop back to them.
They have even talked me through what I have been doing on the phone as I do it when i get stuck or just worried.
If you find someone like this, who will keep your service history going, yet helping you with wiring diagrams, diagnostic code read outs etc when you need them, then your home and dry.