Pool heater install gut check please…

Gut check please.

Got about 5 quotes to install a gas pool heater in Southampton NY (understanding this is prob one of the highest price markets out there). Prices for the heater itself 400k btu plus basic install from existing pipes to connect, cut the old heat exchange up to the house (excluding plumber who head to run the gas line and electrician to power from exiting power lather) ranged from $9800 - 5400. After a bit of vetting I went with the cheapest option as the guy convinced me that he knows his stuff. BUT I need gut check on price and install please for those in the know.

Additional info: Completely new to the game - bought the house end of summer last year. The existing heater was a heat exchange connected to the house’s old oil tank. I learned the hard way one weekend that this can burn through $800 of oil easily over a weekend and without it was just too cold for the kids to enjoy with slightly chilly end of season temps out there. Had national grid come out and run a gas line from the street a few weeks back. Just need to now get the plumber to install the gas line, file the paperwork with t the city to a pressure test and meter and I think (pray) we should be in business.

Side note: I do plan to replace the old oil heating in the house but that’s part of a broader renovation probably after the summer. He said he bought quite a few Rheem units prior to year end to lock in lower prices and could lower his price given it’s still off-season and they haven’t started openings and getting busy so he had guys available. I could have gone Pentair / Raypack as he had both but this is an old liner pool that will probably get redone fully (to gunnite + spa) in the coming years and he says he much prefer these to the slightly more “advanced” alternatives.

Thanks for any help here.