If you think you are unhappy now, that feeling will just get worse as time goes on. Not debating Toyota vs Land Rover, but one is acknowledged as the most reliable (but of course, not perfect) transport option and the other is at the bottom of everyone's list in that regard. Just sayin'.
If you do your own work, and like doing your own work, then you can be happy with any older vehicle that will go where you want, including your Disco. If you are spending $300 per month on your Rover, though, it is time for it to go down the road to somewne else. I don't spend $300 per month on 4 LCs with at least 2 of them daily drivers at any given time.
I think I would avoid a Disco II due to it's more complex electronics, and electronic systems. There are better choices in that price range. It's a fine luxury SUV, but it isn't really a serious offroader in USA trim.
I think that a modestly built Tacoma would do what you want, or, keep the Disco as your hobby/project car and buy a small efficient commuter for everything else. A 4Runner, too, would be a great choice for the reasons mentioned, and not a bad commuter either. I don't follow prices, but I'm not sure it would fit in your price range.
Someone mentioned an 80 series Land Cruiser. It won't get 15 mpg (if modded, 12 is good), but mine has been in the shop one time for a $300 repair in the 10 years (and 80k) I've owned it, and it hasn't been babied either. Not a bad choice, but any older vehicle will require upkeep including any Toyota you might buy. For $20k you could buy an 80 seires Land Cruiser AND a nice commuter ride. The prices on these have come way down, but they are still awesome vehicles and in many ways the best real "SUV" ever sold in the USA.
In the end, I would look around and see what your 4wheeling friends are driving. If they all have Rovers, I'd stick with the Rover. If they all have Toyotas, I'd stick with that. If you go by yourself, I'd definitely get a Toyota.