I have two kids in high school soccer and band and so get an opportunity to grill burgers for events and to make booster money at the concession stand during football season. We use the frozen burgers and with 500 assume you are looking at the same. All I can tell you is those things make a ton of grease. The very best/low maintenance experience has been a rotisserie grill/smoker - something like:
The one we used was home made by one of the dad's - they work great - we were easily able to cook 4 to 6 hours with a load of charcoal (probably a double pack from Sam's). The next best thing was a big offset grill - just cooked in the main chamber and kept the cooked burgers warm in pans in the offset.
For soccer games, we are only cooking about 40 burgers and use just a small gas grill - you just have to go slow and not load it up too much. Once the grease catches fire, things get interesting.
Given the cost constraints of a big rotisserie (I do have the fever for one), I think your best bet would be the Sam's grill - it has multiple burners so you can create a few cool zones to move the burgers if the grease starts to flare up.
Hope this helps.