The most important thing here, is whether the stereo has ISO connectors, and what wiring you already have in the 90.
If you've got a stereo with ISO connectors, and half decent wiring in the landrover, connecting the two is pretty simple. If you're putting the stereo in the standard position most of the wiring should be there, at least for a 2 speaker setup. If you're going for a four speaker setup and/or putting the stereo in something like a cubby box, you'll have a bit more to do.
The neatest solution will probably be to buy an 'iso connector to bare ends' - basically the other half of the connector on the back of the stereo, with 6" of wires coming out of it, which you can join up to the landrovers wiring - then you'll be able to swap and change for any stereo in the future, and it'll work with the same wiring.
If your stereo(s) look like this, you're in luck.
the ISO connector is the bigger one on the left.