I rechassis'd a 200tdi 90 2 years ago, took 8 weeks of weekends and odd evenings. It was summer which helped. I bought a richards chassis which was spot on. Worth bearing in mind richards are used to adding cage mounts and they do some work with North Off Road to add mounts. Depends where you got your cage I guess.
In terms of time three of us stripped the 90 to the chassis in a day or so, the bulkhead swop took some time and was fiddly. I put a different rear axle on, new doors, new suspension etc on at the same time which didn't help. Fiddly bits are lining up the bulkhead/doors/rear tub and the wiring. I gave up trying to feed the rear loom through the chassis as it kept getting caught. P clipped it to the rear tub instead. Plan for lots of small part replacements, allow £500 for bits and bobs that you may as well replace while its in bits, new brake lines, connectors, bushes.....
I didn't think my old chassis was that bad but once removed it was frightening where it had rotted away. I had an Iceland trip planned and didn't want any chassis failure when being snatched out of a river!
The best bit is not having to dread the annual MOT!
Doug