There aren't all that many things that you need to consider when you're going to buy a laminator. And many are pretty obvious: if you need to be laminating hundreds of pages a day you need a much more robust model than the one you'd choose to preserve family photos, for instance. here's a quick checklist.
If you plan to do one-sided (dry) mounting you'll have to make sure your selected model can handle the thickness: depending on your volumes you may well be better off getting a mounting laminator and a regular one.
If you are going to be doing two-sided laminating on thicker material, such as wood or cardboard, you'll have to be sure your model will accommodate it: and machines with four spring-loaded rollers are strongly recommended.
If you are laminating ID badges, or anything else that you don't want to be physically attached to the laminate, just enclosed in it - photos, for instance - then go with a pouch laminator. If the items aren't to be handled much, a cold laminator might also do. But in either case, there is a maximum width that applies: typically 20". Above that you'll have to use a roll laminator.
Fabric may dictate a pouch laminator.
Obviously, the more you plan to use the laminator the more robust a model you'll need. For high volumes, get models with steel gears, four or six rollers, and metal casings. For really high volumes, there are models which can mount a 1,000' roll of laminate. For any reasonable volume, a standby mode is very useful: the machine is ready once it's warmed up for quick use without having to keep heated when waiting for the next user.
You may have a need for one- or two-sided laminations. Most machines should be able to do either. And in principle a roll laminator can do anything a pouch laminator can do, bearing in mind that it will bind the laminate to the material.
Finally, consider durability and repair: better to get a machine rather better built than you actually need - it'll cost more but be more economical over the long run. If laminating is very important to your purposes, look into maintenance contracts or standby machines.