OP...30' 5th wheel most likely will be within the weight range for 350srw, depends on the trailer though as some toy haulers are heavier.
ford 6.4 had emissions issues I had an 08 that I deleted, once that was done it ran great. some of the earlier ones had bad radiators also. If this is your first diesel I would avoid it, step up to the 2011 ford 6.7 or look for a dmax or cummins. BUT NONE of them are trouble free, they all have their flaws it just becomes picking the one you like. 2011 up all seem to be good, my previous 2011 ford 6.7 was a great truck down part was it was a 250.
I wouldn't run from a ford 6.4 if I got a great deal and didn't mind tossing it some $$ to delete it.