We've run Lesco 16hp vacs for the last 9 years, Briggs v-twin - we'd be getting another one if they still had the 16hp; it's a really good unit. Only thing to watch very closely for is don't let leaves accumulate on the deck around the motor; seems obvious, but we've caught fire a couple of times... the backboard on our truck lets strays float back out and they land all over the unit, so we keep an eye on it and just blow it off if they start to gather.
We're looking at the 18/20hp but the weight is getting high for heaving that beast up onto the tailgate 2-3 times a day - we fill our 10 yd dump bed (stuffed and packed) up to 3 times a day in full swing and take the vac off onsite. If you can get the swing-away setup you'll avoid that issue. We have the "over-the-gate" setup so we can still haul a full trailer of equipment too. Two best investments in our business; the dump truck and the leaf vac. Love 'em.
BTW our mower dealer has dropped Little Wonder vacs because of a non resolved issue with bad balance on the impeller/fan which was making the units vibrate to pieces, literally.