Hello Guy,
Forget all the anti caravan crap. Once you've used one successfully you'll find the benefits for we wrinklies, (no insult intended), only serve to enhance the fun you can have when tenting. Done it all in my time and enjoyed every minute.
Damp penetration is the real hazard in a caravan of that age, but it's the way I started and I have even now only progressed to a 10 year old one, to which I've made a few simple but very useful improvements., (including fire proofing to combat TB's attentions).

Have a good sniff inside, and if the mustiness is overpowering, be wary. Rot caused by damp is always far more extensive than you'd imagine.
Leisure batteries come in 80 and 110 amp capacities, and the larger one's can be bought online for around £60/70. There is an argument that they all need a much higher charge rate and may never get fully charged with a conventional charger but I've never had a problem with mine or the onboard charger, hooked up to the mains. Can get a week out of it easily with normal usage, but TV's and microwaves will drain it in a day or two.
It's a pity the hot water heater's been removed. That is a great luxury. Room for a bit of cheeky haggling, perhaps?
Good luck with the purchase. Enjoy it!
Regards,
Bill.