I know few details outside of the CL ad- seller claims over 268K miles, rust, and says that only the A/C doesn't work. Also says it needs CV joints & half shafts. It is a manual.
I know Saabs have a cult following but I think that's based around their turbo offerings- I think this is the 8 valve N/A version, but I am not sure. What's the opinion on them?
What year and what do you want to do with it? Big generation difference between the "classic" 1980-1993/4 and the 1994-1998 900.
Since you mention 8 valve I'm assuming we're talking an early 80's example. Rust is always a concern. I forget which side now, but the front control arm mounts are a common rust point - typically only on once side. There are patch panels and it's a documented repair, but worth looking into.
Paging Mark Wemple if I remember correctly. 
Rust can be real depending on where the car lived. I'm west coast so no rust issues in classic 900s.
My '85 900s 8v has just over 400,000 miles on it and still going strong. Do the axles and drive it. The AC never worked great even when new, but I imagine if you know your way around these things you could do some modern refitting.
Wonderful cars - the 85-ish years had the biodegradable insulation wiring harness that was all over Europe around these times. The 8v are all CIS engines. CIS HATES sitting, so if the car has sat a while, expect it to be cantankerous for the first few weeks of driving. Change the fuel filter and drive it.
I probably have the least desirable 900 model as far as the cult following goes- a 91 900s 4 door NA. And I really like it.
In two years of ownership I've changed the oil, replaced a brake light switch and fixed a vacuum line. My AC hasn't worked during this time, but whatever.
From what I've read the rust thestig99 mentioned is on the passenger side, and is more prevalent in turbo models.
If the one you are considering has ABS, don't be surprised to see an ABS warning light on the dash when you step on the brake. At least in my 91, that system included an accumulator- a metal bulb with a threaded end on it- and they fail internally. But the brakes will still work with a failed one. Replacements are pricey.
As with most cars of this age (probably) the plastic stuff starts breaking. The fuel filler neck, one of the seatbelt latch housings, the dash (lol).
But they are cool cars- at least to a wierdo like me they are.
Running C900 for 600? Go get it. They're quirky cars, but parts are still available, and there are documented procedures for pretty much everything, including repairing the front half shaft tunnels.
I'm far from an expert, but I do have a rust-free one sitting waiting for me to actually do an auto > manual conversion and start driving it.
In addition to the other possible issues, the trans was an issue, more so on the turbos.
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