Simon, where do you plan to use the RIB? The Attaque doesn't have a deep V hull, and wouldn't be the best choice unless you'll only be in sheltered waters. On the other hand, it doesn't need a huge engine to get a decent speed and should be cheap to buy and run.
I would guess that a 20 year old, shallow V RIB needing some TLC would have a value in the low hundreds, assuming no major faults. Maybe worth a grand if in decent condition.
Without a diagnosis, the engine is worthless. Maybe worth £500 if it's a runner.
Trailer is probably worth a couple of hundred quid. I assume it's unbraked which means less to go wrong, but expect to replace the bearings at least. You may need to replace the entire axle/hubs. Make sure the rollers are all OK if it's a roller trailer.
If it was all in sound but scruffy condition, I'd guess at around £2000 total value. As described, I wouldn't pay more than £500.
Having said that, any boat is worth exactly what someone will pay for it; not a penny more, not a penny less!