Drywall – How likely is it that the 1980 home has asbestos in the drywall

My thoughts on your first question would be to take a sample from a few different rooms, if you can see any differences between color/texture on the ceilings. From asbestos.com:

If the material looks different (especially in color or texture) in different parts of the room, plan to sample each of those areas. For a ceiling surface in a large room, two samples may be needed to determine your asbestos situation. Try to balance the need to protect your family from exposure after you sample (to sample in an inconspicuous area) with the need to get a representative sample of the material (to sample in the center of the room).

I'm not sure about the size of sample required, but I don't think you'd need more than a few square inches.

As for patching the ceiling, you should prime the sample spot before painting. After that, you can mix some standard ceiling paint with a texture additive to match the consistency of your original ceiling. Since it's a small area, you can probably get away with using a brush, or possibly a small roller. Or, instead of bothering with texture matching, you could go the easy route and use spray-on popcorn ceiling texture after priming (beware the horrible product reviews though - it might be easier to use a pre-mixed product).

As a certified home inspector I will give you some more specific advice from a Buyer's point of view. If the inspector suspected asbestos, then this warning should be taken seriously. At this point in the house buying negotiations, it would be wise to insist the "Seller" have the material sampled by a licensed tech and tested by a certified lab with a copy of the results given to you before proceeding to a closing. If the tests do come back positive for asbestos, get a quote from a licensed asbestos remediator to remove it, and a quote to replace the ceiling with some safe material such as drywall. I would recommend asking that the total amount of both of these projects be adjusted from the selling price.

Under no circumstances would I advise one of my customers to proceed further into the purchase until this issue is resolved. If positive for asbestos, this could be an expensive fix and definitely not a DIY project under any circumstances.