Biggest thing is to make sure it has a fan, not just a passive heating element. The fan will decrease your drying times and help all the trays dry evenly, although rotating trays is still advised and usually necessary if you stack more than a few trays on a vertical system.A thermostat is also nice to select different temps. For a timer I just buy a regular outlet timer for a generic appliance.
The horizontal ones where the trays pull put of a larger box are more expensive but not as scalable - you cant just buy some extra trays and stack em on to make it bigger, but I think they do dry more evenly. I've never used one. The Excalibur is the most common and cabelas has some "commercial" ones of this type too.