First of all: Do it. These are some of the most usable classic cars in the world.
25 of the 107 cars I've owned have been C900s. Our current Saab fleet consists 2 C900s, a 95V4, and a 9-3 SportCombi registered and in regular use. There are four C900 parts cars at the back of the property, hidden in the woods. We have some more (and less) normal cars too, but that's not what this thread is for.
There is absolutely nothing that would scare me about one as a limited use daily driver. They are robust and fairly easy to work on, but certainly different from other things. You'll easily break your 20MPG number with anything other than an automatic 8-valve turbo. The 16V n/a manual cars will get the best economy in my experience.
I've killed one automatic and one manual transmission in them. Both times I was being a complete shiny happy person, but they're definitely the weakest link. There is a well-regarded rebuilder of C900 manuals in IL who will ship one to you for under $2k, so I don't really sweat it between that and my parts hoard. The trans is an easy weekend replacement if you have the parts ready to go in.
I'd be interested in hearing what parts people are having trouble finding. Trim stuff? You're mostly looking for used parts, though there are some people (Saabits in the UK, for example) who are stepping up with some of the more commonly damaged trim like emblems and fender moldings.
I'd buy one based on condition versus options (with the exception of the trans-- you probably want the manual). I've had more fun with n/a 900s than turbos, but I'm definitely in the minority with that statement. Track days, rallycross, cross-country trips, Deal's Gap, autocross, whatever. They're fun and they'll usually take the abuse in stride. The turbo cars are more needy but I certainly wouldn't avoid one and may prefer one depending on usage. Still, condition is king. Find one with decent interior and an excellent body and it'll always be worth fixing.
The classic Saab community is the best marque-specific group that I've encountered. Parts, information, or even roadside assistance--- I've personally gotten all three. It is very GRM-like in that regard.
If you have any more specific questions, post up. There's a lot of Saab experience on this forum.