These cars are pretty well engineered and lots of the larger maintenance jobs you would expect of a 20 year old car are within the remit of a DIY owner.
You will struggle with corroded fixings almost everywhere underneath the car. The access to the 911 is also pretty difficult unless you have a lift as most things are done from underneath. If you don't have a garage I'm not sure I would attempt too many tasks, although you might live somewhere less wet and windy than Scotland in which case driveway maintenance might be an option!
You will also find that the first couple of jobs you do will cost you almost as much as taking it to an indy as you will be buying some tools and equipment to complete the task (unless you already have a stocked workshop). You will need axle stands, a head torch, a minimum of two torque wrenches, a breaker bar, a couple of socket sets, torx bits, lots of wobble bars and joints, channel locks, mole grips, blow torch, punches, etc. Then there is the penetrating oil, ally grease, stainless jubilee clips, etc. Lots of these are inexpensive by themselves but could easily add up to a few hundred or more if you buy all the tools new/at once.
The positives, however, are that you will be able to source all the parts yourself and satisfy yourself of their adequacy and quality. If I was taking the car to an indy I wouldn't expect that they would be able to take the time to pick particular manufacturers for each part, they will take what they can get from their factors delivered same day. Ramp time is costly for them so unless you want them to order all the parts from the OPC (which is an decent if expensive option) the doing it yourself allows you control over this. You will also be able to take your time and fix all the other little bits and pieces around the job site that an indy would not touch as part of the job.
You will spend as much time on the internet researching how to do a particular task as actually doing it! This forum is an excellent resource and the members are very helpful. Pelican Parts also have lots of good articles with photos. Here is a link to the Porsche Workshop manual for the 996:
http://arma.free.fr/porsche/996/GT3/996 Group 0 General.pdf
Finally, you will get to know your car well and owning it will be more pleasurable, for the simple reason that when something goes wrong you will be armed with the knowledge to fix - or at least diagnose - the problem yourself.
