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Storage 6months - cause for engine sludge?

interested in buying a 07 Saab 95 aero sportscombi from out of state. Been getting maintenance history from owner and found out the car was stored for 6 months in CO without draining the oil or driving the car every once in a while. Older model saabs are prone to sludge due to a design flaw. Would the inproper storage cause engine sludge in the long run? What are signs of engine sludge and should I ask a mechanic to pull the valve cover to check for sludge during pre-purchase inspection?

Sitting for six months would not–in and of itself–be a cause of engine sludging.
The causes of engine sludging are:

Not changing the oil often enough
Allowing the engine to do a lot of idling
Doing a lot of short-trip driving, particularly in colder weather
Not using the proper specification motor oil

When you say that you have been getting the maintenance history from the owner, I hope that you mean he is producing documentation of its maintenance, rather than simply telling you information that might–or might not–be accurate. Personally, I would never buy a used car unless it came with full documentation of its maintenance history, but even then, I would still have it inspected by my mechanic prior to purchase.

As to oil change intervals, bear in mind that the extended intervals seen in most maintenance schedules are “ideal world” schedules. In reality, most cars are driven in such a way that they need to be maintained according to the Severe Service maintenance schedule that should also be in the Owner’s Manual, even if it is not as prominent as the “normal” maintenance schedule. Personally, I would never go longer than 6 months between oil changes, but for some cars even that is too long. If the car was subjected to a lot of short-trip driving and idling, a 3-4 month oil change interval is more realistic.

If there is any question as to whether this engine might be “sludged”, removing the valve cover would be an excellent idea.