Well I won’t touch the heater then, and I asked if I should apply an underseal and I said it was a 110
Underseal is used a generic term for rust proofing the underside of a car, however underseal as a product itself is a terrible product that seals water in and allows the chassis to rust away behind it while still looking in good condition. similar to powder coating. It should be avoided at all costs, never applied to anything and if someone has already applied it then I agree with the above regarding walking away.
If when you say "apply underseal" you actually mean a good quality chassis wax then that is a different story and should be done inside and out on the chassis, bulkhead, door frames, anything else steel as soon as possible and then every 1-2 years after that. I personally like dinitrol, but know others here have had good results with lanoguard. Waxoil used to be better than it is today, but is still better than nothing.
I’m getting a 1987 defender 110, is there anything I should do to it as soon as possible to make it last like sealing the underside, and I’ve been told to take out a plug in the heater wing ? Any other tips or thoughts !
In terms of things to do as soon as you get it if you don't know much about them I agree with the above about getting someone who does to give it a look over whether that is a friend of the local independent land rover specialist. Things I would be doing immediately unless I knew it had just been done:
Full vehicle fluid and filter change including all transmission, brakes, etc.
correctly adjust the hand brake and rear drums (lots of people never do)
Timing belt
Thorough check of all suspension bushes, wheel bearing and UJ's with replacements of any suspect parts
"underseal" (apply chassis wax!) to everything on the underside
Fully inspect and repair all vehicle wiring. (I am a little paranoid about this as I had an electrical failure start an engine fire a couple of years ago necessitating a full rebuild. People like to bodge wiring horribly with terrible connections, additional wire, and overloading things)