HH
For the very commonest aircraft, e.g. C152, C172, Cherokee/Warrior/Archer, just about any maintenance organisation will know these aircraft inside-out and will be able to do a good pre-purchase inspection. But for anything more unusual and I still mean really common stuff like Arrows, 172RG/177RG, Beechcraft, Jodels, Grummans, etc., it is worth seeking out an engineer who knows the type well, as the sort of expensive problem that might ruin the experience is likely to be well hidden.
I guess you could you ask how many annuals they have carried out in the last 12 months on that particular model. You could also ask to see their pre-sales inspection checklistl - many maintenance organsisations have a generic pre-sales inspection checklist. And I guess you can ask them what they will specifically look if you hire them to check a particular model.
Quote "Would an inspection be economically viable if the share is less than say £2,000?" You are looking at this the wrong way around. The inspection will be "economically viable" if it saves you spending the cost of the inspection on fixing some otherwise unseen fault. (Of course the cost of fixing the fault would be shared, this being one major advantage of sharing!)
In some cases, the cheaper the share, the more important it is to have an inspection - the share might be very 'cheap' for a reason, e.g. the other shareholders know or suspect that a big bill is on the way.
tp