definatley belt driven if you plan on doing any cheese, and at least a 12" blade. i have a berkel and it works great. .3 hp and a 12" blade. it comes with built in sharpening stones (so you dont make the whole kitchen cry when you are slicing onions)
but they are a pain in a$$ to get really clean. we slice meat, cheese, lettuce, cabbage, tomatoes, and onions. all that junk builds up on the guard and you have to use a socket wrench to fully remove to get all the crack and crevices. i swear, i wish someone would come up with a easily removable blade guard.
slicers can be a huge health hazard if not cleaned properly and health inspectors love to check them first (after they have checked your dumpster door flap!)
i’ve only used berkel, but other reputable brands would be hobart, or anvil