Get the biggest one you can possibly afford. If possible, then get the next size up.
Same with depth - you'll be wanting to do pot melts before you know it, and you need the height.
ABSOLUTELY get an electronic controller. Preferably one that has digit keys, not just an up and down button to set the temp. At least 4 user programs so you don't have to be changing it every time you fire.
Window is good. Lid lifter is good if you go all out and get a really big one
Elements in the lid actually are better for fusing (otherwise it heats from the sides and the edges fuse first, causing bubbles to be trapped.) Mine lets you select.
Top loading is good if you ever want to do raking.
1700 minimum for a high temp, preferably higher so you don't have to worry about killing elements doing said pot melts.
I have three kilns at this point. A paragon fusing/annealing kiln, which is long, has a clam shell lid, and 2 bead doors. That was my first one - it's great for beads, and is what I start fusing in, but quickly outgrew. Even works for enameling - at least the first side, since you can use the bead door when it's just a rack, but hard to open the lid, remove the trivet, close the lid so you don't lose all the heat, without dumping the enamelled piece somewhere along the way. The controller is one with just the up and down button for setting, and it's a pain in the butt to change. Goes up to 1800 I think.
My second kiln is a Jen Ken 25" octagon, 15" deep. Will hold a 24" square piece, and has a single shelf, which is really nice (and somewhat unusual for a kiln that size.) Has elements in the sides and in the lid, you can select which you want to use (or both), and can be programmed every way from Sunday with a very nice Orton controller. It is however a big sucker and my dh had to put in a new panel board in the garage for it, and it's definitely not house current.
My third one is a Paragon front loader, 8x8x8, purchased for enamelling, although I may try some smaller fusing pieces in it. Elements on the sides only, controller with the up and down buttons (not such a big deal on this one since all I need it to do is go to 1450 and hold), but does have several user programs. Runs on household current, same as the first one.