Yeah! $6600 for just the rebuild, plus re-install yet; sounds like he's tryin' ta sell ya a new motor (or a bill of goods !). $4600-4900 for the MMI and IIRC, ya get a core chg back? I had a large Onan 4 cyl genset rebuilt a coupe years ago to near the same extent and it was only $1100 ! And Onan charges near Harley prices for parts!
Conversely; find a solid used A-4 ..I've seen some in the 1200-1700 dollar range...and have a pro go over it. Tweak as needed and yer into less than $2500. Pop it back in yerself and it's all good
While I'd opt for a diesel; the change over costs would be prohibitive, so stick w/gas and go, go...go !