Buy large enough. I discovered that the largest tractor I've used leaves the least noticeable tracks in the grass. A cab is wonderful, and A/C really makes life even better.
Yeah AC is very tempting. The one positive is on most of them you can take the doors off, so during the summer which I dont plan on mowing with this, if the doors are off it shouldnt be bad. IVe mowed my whole life without a cab and no top cover.