If you are seriously considering the car, I would recommend a Pre-purchase inspection by a qualified shop. 17K would be on the high side of the price range for an SC. So for 17K I would want a car that was in very good condition. Get the PPI and see what it says. With cars this old there are a lot of little things that can go wrong even if it just sits. Rubber brake lines wear out, the bushings in the suspension, etc. etc. Get to know what is worn out, what needs replacement, and an estimate on how much it will cost to repair anything that is wrong with the car.
If your price range is 17K I think you should shop around a little bit and experiece a couple different cars and find the one that is best for you. Maybe it is your friends car maybe it isn't. Check your local Porsche Club of America web page there could be some decent cars advertised there. Just be certain to get a good PPI with compression and leakdown tests on any 911 you are seriously concidering.
Here is a link to your PCA website:
http://cirpca.org/Home/Default.aspx
And a link to the PCA home page:
www.pca.org
You can become a quest member of the PCA here:
http://www.pca.org/join/quest.html
That will allow you to look through the mart and see what other cars are out there.
Good luck on the hunt, at 17K you should demand the best.
Rich