Maniac, like the Caseman said, all on your list are good ones, but then again, ALL farmalls are a good choice!!
I like the letter series myself, but that's mainly to set a limit on the collection.

The 400/450 would be quite simple to work on, pretty straight forward battery ignition, and basic internal combustion gas engine. Good solid units! As for price of parts, they can be pricey for ANY tractor. Ussually there are enough Farmalls around that used parts are availlable. One thing to consider though, is the hundred series and 50 series were only made for 2 years each, so parts specific to those models might be a little harder to get used. Take a look at tractor salvage yards near you.