Hi David and welcome to the club. Without some pics of the boat it is difficult to suggest any upgrades. However, assuming you are going to physically see the boat and will get a good sea-trial, I don't think I would waste my money on a full survey. The boat is 24 years old, so likely a surveyor will find lots of things to worry you, most of which will have no real effect on your use or enjoyment of the boat. Your big concerns should be Is the hull sound/watertight? Do the tubes hold pressure ovetr time? Is the engine functioning properly?
Most of those things you can check and observe yourself. However, it would be useful to get an engineers report on the engine (compression test, etc) that should cost you about £50.
Anyway, good luck with the purchase and I look forward to seeing some pics.
You get what you settle for!