Welcome to the forum! There are a number of LY MH owners on here that can give you specific advice. All of us MH owners share the P30 chassis and 454 (albeit a few diesels and Fords) and most of us Classic owners have units far older than the 91 your are looking at - so you coud say we all specialize in age
Check out Fred's Airstream Motorhome Website (www.airstreammotorhomes.com
) for some pictures and information on all of the AS Motorhomes. Also try searching the forum from Land Yacht for more information.
One thing to be extra careful with is the condition of the engine/transmission. It's likely that it could need replacement, not because it was absued or over used but rather because it was under used. Engines, Gensets and Trannys like to be run often and lack of use or long term storage can lead to premature failure even with low mileage. Make sure you get things checked out by a mechanic and let him or her know about length of time this unit has been sitting. Also ask the owner what maintenance has been done and how often they run the unit. The fact that it needs tires is not a good sign in that it probably has not been run on trips for quite a while.
I'm sure more will chime in on this topic as well.
Of course, depending on the condition and maintenance you might have found a real gem. Make sure everything works (genset, fridge, airs, heaters). AS's are typically very well appointed units and were the "top of the line" when new. Many of us keep much older units than the LY's on the road year after year and the maintenance can be quit a bit, but we get to enjoy the benefits of a high end coach for a fraction of the cost of a new one.