Ok, I love the idea, and its something I myself would like to do. However, your approach to this is not really the best. Long travel IFS/IRS lifts are great on a desert truck that your budget for suspension and drivetrain is around $60K alone. On a RR you intend to only spend $15K building, not so much. I dont know how the front and rear of the underside of an MKIII look, but chances are its like a unibody jeep, and an IFS toyota, aka there will be room to fit links. You are going to spend stupid money on the custom lift arms and coilover mounts, and in the end they are still not going to flex like a real axle. If I were you i'd get some sized Toy 8", Ford 9" or D60 axles for the thing. From there, go 4 (without panhard) or 3 (with panhard) linked rear, and 3 or radius arm linked front. Then put some 12-14" coilovers up front, and get spring buckets made for the rear, and run RTE springs back there. A RR airbag is about the size of a coil, and spring buckets for coils will be cheaper than CNC'd coilover mounts. In the end, you will probably end up spending $15K on just the solid axle conversion (axles and suspension).
Why am I telling you to do this, for the same reason you dont run 35s on a D2 (for now) and you dont keep IFS on your tacoma if you want to wheel hard. Youre thinking outside the box by wanting to mod an MKIII, but you need to think further outside it. Because in the end, you will have spend an obscene amount of money on this independent lift, and what do you get. A taller truck that flexes a bit better but can still be put to shame by an OME equiped 1988 RRC with 32s. If you do this, build it right, make something that puts almost every other rover to shame. In the end, with your proposed plan you have now, you will still have an MKIII drivetrain platform, this means you'll have axles that will break under 35s and replacements arnt cheap, you have diffs which dont have lockers available for them, you have weak unitbearing hubs, and you have brakes that cost a kidney to do a rotor and pad replacement. You'd be much better with some custom solid axles, theyre alot stronger, cheaper, and way more functional.