Best advice I can give is buy the best you can afford, it's hard to go wrong that way.
I've got three myself, an Atlas Industrial Rail Public Service of New Jersey trolley (came with a set now out of production), an MTH NJ Transit PCC, and a limited run Virginia Train Collectors trolley made by Bowser. The Atlas set I got new at $150 about ten years ago, the MTH PCC for $75, used, at a train show four years ago, and the Bowser at $80 a few months back. At the prices charged I thought they were too good to pass up. All good and fun to run, and a trolley does add a nice bit of extra action on a layout.