150k is fuck all for a quality motor. If it was a Fiat, Ford, Vauxhall or Renault I'd maybe be slightly more concerned. If I was a Saab owner what would concern me is "sludging". I believe some Saabs have issues with "sludging" where the engine oil distribution is poor due to a sludge like build up at the bottom end. Some parts don't get lubricated enough and as you can imagine that will cause a fairly monumental problem eventually. This is such a problem that the tendency for people (who are aware of the issue) that buy a used Saab the first thing they do is drop the sump and clean it up. However I honestly don't know if cars as old as 1991 900s are affected by this issue.
There's also an issue with the bulk head cracking because the steering rack is bolted to it and it puts it under extra strain. You can buy kits to reinforce it which consist of two chunky metal brackets which is meant to prevent it from happening. Still brings a whole new meaning to the term "bombproof" though.