If you have the money, a new chassis is a great option, but I have seen many replaced when some much cheaper repairs could have extended the life of the original considerably (10 years+?). As everyone knows, the rear crossmembers are notorious for rot, so are the front dumb irons and outriggers. Later chassis also seem to suffer rot in the middle too, but I generally find that a lot of earlier ones don't, so if the extremities are sorted, you can still have a good structure. My '89 had a patchy rear crossmember, but apart from one area on the front iron (3"x2" metal cut out and new welded in), it was sound, so was very happy not to replace it, keeping originality and proof of ID, as chassis number is kept. Have a very careful look around the rear trailing arms mountings, the area around the big bushes. If there's rot there too, I would probably look at replacement. Whatever you decide, make sure it's very well rustproofed, inside and out to protect it from further decay.