Look for a basement with some ceiling height. You may think just tall enough so you don't hit your head is fine, but even more than floor space, ceiling height will impact your choice of milling machines. Taller walls also add up to more storage space, more space to decorate etc. An 8 foot tall shelf is 25% more storage than a 6 foot shelf.
You don't want to know how many 2x4 foot pieces of sheet rock I have due to 6 foot walls...
An outside door is one saving grace of my shop, only one step to deal with, and I don't have to bring stuff through the house. It is a small door though, very tight for some of the larger machines. Also nice because walking around the house to go inside provides time for chips to fall off and I have immediate access outside for ventilation (just open the door) and dirtier work. I have a workbench set up just outside the door for grinding, as well as an area for welding.
By square footage I am about equal to a single car garage, but I do envy the ceiling height and large access door of the guys with small garage shops. Then again I think I have basement shop in my genes. Something very comfortable to me about a cramped space in a basement. I had a model bench under the basement stairs as a kid, loved that.
Electrical service is something else to look at particularly if an older home. I'm in an old house which could have been a problem, but it was used as an office for many years so they updated all the electrical in the house including a 175 amp panel.
I'm a fan of sheet rock for walls, although I can appreciate the screw stuff anywhere ability of plywood. Sheet rock is cheaper and more fire resistant, both of which were considerations for me.
As far as floor coverings, I went with vinyl flooring. It comes in strips that fit together like Pergo, is cheap and very resilient. It is used in many commercial buildings because it is tough, cheap and looks decent. I think I paid around $1 sq/ft, it was easy to install too. I did mine about 2 years ago, and it looks about the same as when I first put it in, so it is either holding up, or I don't spend enough time in the shop (the second is true so maybe some of both).