First before you do anything, you need to decide if you want a machine that will pull from the tank or if you want to connect a garden hose from the house each time you use that pressure washer. By mentioning that you have a 325 gallon tank in the trailer, I am guessing that you would want a pressure washer that can pull from the tank so you only have 1 hose from the house to the trailer (unless that house has poor flow, then you need another hose).
Most direct drive pressure washers out there will not pull from a tank, some will but the majority will not and I am not sure if you want to take the chance on buying something without knowing if it will or not. Russ J. at southside can tell you if you have the info from the pump (you might be able to get that from the person auctioning off the machine, maybe).
Dewalt does not make pressure washers, they have them assembled using pumps, engines, unloaders, frames, etc.... then put their name on the machine....which does not matter one bit as it is a lesser-quality machine than what you would buy from one of the vendors here. Those are throw-away pumps most of the time meaning that the cost to rebuild it, it is cheaper to just buy another pump to put on the frame/engine.
I would first decide if you want to connect a hose to the house and machine each time you use the machine (what do you do if there is only 1 water faucet on the house, it is a big house with lots of work for 2 machines and you need to connect the tank to the house also? ? ?
They are right, for a little bit more money (I think some of the vendors have machines on sale right now) you can get a belt drive or gear drive machine that will pull from a tank (you need to mention this to the vendor so you get the right machine) and you can connect the machine to the tank (I would put a ball valve there before the connection to the machine so you can take the machine out of the trailer if needed) and be able to take the machine to other parts of the huge house if needed for convenience or speed.
Good luck.