I'm halfway though this job myself but on a older 2.5td 90, so you might have additional stuff because your model is later. You need a number of parts as mentioned, the steering box, irrc the rod the steering damper connects is handed, the dash but not the cluster/clocks/column, the handle for the other end of the dash and the heater/fan assembly is handed. For brake lines, it was time to change them anyway, so I didn't check, just cut them off and will make new to suit the new positions.
Not sure on wiring, the wiper drive cable + wiper arms, and the front axle hub castings are different. The bulkhead I punched out the popouts on the other side and plated over the rhd positions but the clutch/brake/throttle swap sides, although there's a support bracket mount for the column you have to move side too. I think if your dedicated you can rework the rhd dash and heater assembly to lhd versions with cut and shutting, but its not a simple job, far easier to just swap over to lhd versions if you can find them. Of course you need new headlights as the beam pattern is wrong if your registering it elsewhere. I kept my original mph/km speedo in lhd land for over a decade now and have had no problems with inspections/the law .
I took the route of buying a dead rotted 200tdi discovery1 to get the major parts, and I've used the front axle and steering box from that, and I'm converting from 90td to its 200tdi while its all in bits. I could probably swap just the axle castings, but this seemed the quickest simplest way to achieve it. I was going to use the springs and dampers from the disco as they were new and the 90 ones I have are very tired, but they're softer according to specs, so I'm fitting new dampers and putting my 90 springs back on after blasting/paint but swapping their sides. I'll worry about the dash/heater when I actually get to that bit, but there's always used ones in the small ad's here. I fitted new spring turret tops because predictably they were manky once everything was stripped.
I say halfway, because when I stripped it, the chassis was in poor shape in hidden parts once the body was off, so to save time (!!!!!) as its such a big job my wife persuaded me to replace the chassis rather than repair it, and since got delayed 7 weeks waiting for shielder to ship a chassis out to me in europe, and its turned up today and they've shipped me a 300tdi one for a 200tdi. So I'll be lucky to contribute more from experience this side of winter given current delays and problems. Yes I should have just sorted it myself but for once I decided to just do the right thing and buy one done and galv'd ready to por15 and be done. But its worth considering that when you take your car to this many bits, there will be lots of stuff you want to fix you never really was aware of before. But it needed doing anyway is my philosophy.
This may not make sense to everyone but I've owned my 90 for nearly 15 years now and I'm kind of attached to it & while its in bits getting a chassis etc, its a good time to do it. And it'll still be my 90 after.
If you search youtube there's a series of videos on it by britanica restorations.