Hard to find parts would be body parts and interior parts. The engine parts are common with other Cadillacs with the exception of the manifold which is specific to the Allante I think. The parts are out there, but you will pay a premimum for them. A example would be the hood made of Aluminum at about 2,000 to 6,000 to replace. If you get one, be sure to insure it well as many insurance companys will total them even for minor damage. The good part to that is if you have a policy for replacement value, you could get a replacement Allante and buy back the wrecked one and fix it.
Don't let me scare you out of buying a fine car, just do as much homework as possible before you buy as with any car purchase. Tell me more about the car. Exterior and interior color, digital or analog dash, hardtop, and if the owner is the original owner or is it from a dealer. There may be a member of the AAG that lives close egnough to you to even offer to go look at it with you. Chigaco and St. Louis have very active Allante groups, and you might get some info from there web sites. Spend some time on the web and do some research and lots of reading about the car. They are great cars, and not alot of them out there. Most owners of Allantes are very helpful in offering info and advice on there cars and I think you would be very happy as an owner. The price and miles on the one you mention sounds almost to good to be true. Look at Ebay to get an idea of what they bring. Do a search of completed items to see what they sell for. I bought mine off Ebay, and was very pleased with my purchase. You might also want to take it to a dealer to let them look it over. Again look at the Allantenet.org web site to find a good dealer in your area or an owner near you to help coach you.