Thank you all for such genuinely helpful advice. And I'm impressed the thread didn't degenerate into another LAA/permit/CofA debate!
The "buy cheap buy twice" advice rings true, as does "you'll find it's a lot easier to buy a plane and then form a group" although these two things mean that I might be looking at spending more than I had wanted to. The info on ADs, SBs and log books is very useful. I've come to the conclusion that I will need to take someone along who is more knowledgeable than me.
To those that were asking about mission and aircraft type, my original question was more about HOW to buy an aircraft rather than WHAT aircraft to buy...but I'll expand on that anyway:
I'm a very low hours, newly qualified PPL. <This is where everyone groans and calls me a dreamer> My main desire for a share is flexibility and availability although a lower hourly rate would also be nice but isn't as important. As the moment I don't have a "mission profile" as the majority of my flying up until now has been training. What I would like to do is gain some more experience by flying to places I haven't visited before. At some point I'd like to fly across the water (Channel Islands, Ireland and Europe) because, let's face it, I can drive over land. Having the flexibility to visit places and stay for a few days is difficult when hiring club aircraft.
I want to get myself a share in an aircraft so that I don't fall into the rut of flying to airfields that are 30 mins away from base for a sandwich. I'd love to join an existing group as that would be much easier but I've yet to find a suitable one locally. There are shares around but they seem to be in faster, complex aircraft and I don't think I'm at that stage in my piloting capability yet. Either that or they're based further afield. There is the added complication that some groups won't accept a low hours PPL despite my willingness to accept the higher insurance excess.
I'll keep looking out for an existing group to join while investigating purchase options. When it comes down to it, the advice here seems to suggest that I'll end up spending more than I had intended