I own an old Defender 90 (as you might be able to guess from my user name). It's used off-road regulary (we live off-road) and for farming duties.
If you really *need* one, then you'd probably be better off getting a L200 or a Toyota; these are gradually replacing the Defender as the farmer's vehicle of choice.
If you *want* one, then go for it; basically it's the biggest Meccano set you've ever had in your life. But you need to be realistic about it.
It won't be cheap - old Defenders hold their price amazingly well.
It won't be comfortable - they're very small inside and pretty basic and driving any distance on tarmac will leave you cold, shaken and deaf.
It will need regular tweaking - you'll need a basic toolkit, e.g. angle grinder, MIG welder, hammer, large hammer, very large hammer. Spares are easily available though, fairly cheap and easy to fit.
It won't automatically go anywhere - you'll need to learn how to drive it properly. Quite a few Landy off-roaders get to join the upside down club.
On the plus side, it's just the greatest fun vehicle in the history of automobiles.
You'll either love it or hate it, and there's only one way to find out.