The first tractor (round fender) doesn't have the original motor, and I think is way over priced. Looks to be a 1966 110.
The second tractor will need some work, but again, I think over priced as well. Guessing the year of this tractor to be 1971/1972.
The third tractor would be a good project and is priced reasonable, but like Travis said above, I'd give an offer first.
I realize you're wanting to stay local in finding a tractor, and I realize your son is wanting one badly, but sometimes it really pays to be patient. Don't pay or settle for something that is going to be nothing but problems later. Eventually you'll find a good tractor that won't need as much work as the ones you have listed, and will be in the right price range. These first two tractors are well over priced for the amount of work you're going to put into them.
Not sure if resell is on your mind or not, but it should be. If not in your immediate future, possibly your son's some time down the road. You do not want to spend a lot of money on buying a tractor, put in a major amount of time and money, and then not be able to get your money back out of it. Restored, the most you can resell the flat fender 110's for, is usually $1000-$1200 tops. Restored round fender 110's can be sold for a little higher, usually between $1200-$1500, but they must be done right.
As for attachments, pricing depends on which type of attachment you want, and they overall condition of each attachment. A round fender rototiller could cost anywhere between $75-$150, whereas a flat fender rototiller can be bought between $100-$250. Not much difference between the tow models other than the mounting brackets for each type of tractor. Mower decks are still able to be found and will run you between $50-$250, again depending size and condition. Other attachments to thing about would be a front blade, snowthrower, or dump cart. Again, depending on condition and which type of tractor you decide on getting, these items could cost anywhere between $50-$200 for a round fender, and $150-$400 for a flat fender.