Life of Ex MOD Rib's
Whilst I can advise on the tubes I don't profess to be an authority on the actual hull's.
In my experience the majority of the hulls that I have seen, have always been in excellent shape, and have gone on to give many years of good service. However I am mainly refering to P22, A22, and A28's built by Halmatic. Having seen many hulls being constructed by them over the years, I doubt whether you would find a more robust construction. One word of warning though, is that a batch of hulls built in about 1991 could have been built by the now defunct Victoria Marine, and I know that the MOD did not go back to them for a repeat order. They reverted back to Halmatic.
As for the tubes, despite what they may look like, they can normally be repaired. Things to watch out for are whether the inner baffles are intact, and obviously any major leaks. All Seariders and Halmatic's are built with Hypalon coated fabric tubes and can easily be repaired. If however the tubes are beyond economical repair I know a company that can supply new ones!!!!