Hey man. That is a great engine, and the unit can make you money, but for just a little more you can get one with a 5.5 GPM pump… which is what a lot of guys with the GX390 4 GPM end up upgrading to anyway. That’s what i did.
I’m showing $1300 for the Dewalt, and here is a belt driven 5.5 2500 for $1475.
Belt drive is always better than direct drive… several reasons why… mainly because it’s better on the engine, and it can easily pull water from a buffer tank. Direct drive sometimes has issues pulling from buffer. The third option is a gear driven pump, and those are about the same as belt driven… maybe slightly less desirable, depending on who you talk to.
I know you said that you were doing it through HD so you could use a credit card, but you might be able to do the same at pressure washing vendors… dont know.
The trade off for the increased water flow, is that the maximum PSI drops from 4200 to 2500, but the only time PSI is really relevant for us, is when doing concrete (or if you were running super long lengths of hose, i suppose). 2500 is sufficient for residential concrete, but commercial can sometimes require a little more… with 3000-3500 PSI typically being recommended for that type of work. Still… nearly everyone (if not everyone) says that the water flow upgrade trumps the PSI loss, because it speeds up all of your jobs (rinsing is what takes the most amount of time).
Hope that helps, keep searching and reading forums like this, and you will be fine come spring.