Many of the older RVs will have non-functioning cab ACs - they can be very expensive to repair. You learn to work around it. With mine, I use a 12v fan in the back to help with air flow while on the road. I have a fridge with nice cold drinks as needed. And I use old fashioned AC - open the cab windows.
Don't worry about resale. You're buying an RV so you and your husband can enjoy your time together - not so you can make money.
Even though it's winterized, make sure you test the water, etc. It's too easy for someone to say everything works when they don't have to prove it.
You mention he's a neighbor and that he likes fixing RVs. Would he be willing to help/advise you with repairs, etc. on the RV if needed? If so, I might be inclined to offer closer to his asking price for the goodwill.
With an older RV, I do recommend getting the fuel lines and propane lines checked. A leak in either could be disastrous or worse and it's an easy thing to check.