its an inverted flare fitting.
if its leaking, either the tube flare is bad, or the inlet fitting in the carb is bad.
easy fix, either replace the tube nut and re-flare the end, or replace the inlet fitting.
THERE IS NO SEALANT OR SEALING WASHER.
welcome aboard.
there are other boats for sale. offer him what you want to offer, then walk away. they will call you in a day or two if they want to make that deal.
Anti bond 2015 or a razor blade
https://www.westmarine.com/buy/anti-bond--anti-bond-2015-1-2-oz-spray--P001174804?pCode=1174804&gclid=Cj0KCQjwwJuVBhCAARIsAOPwGASQ6oLgRA4JVPA6r3AnkH275edlEBz4j83Je7Sl8WxEWmoEhjBJmNgaApoVEALw_wcB
You wont find hubs to match your current suburban wheels. You would need adapters, etc
The 13" wheels/tires are not limiting your ability to go off-road. The trailer frame, springs, and the bunks/rollers are the limit