I think that, given the choice of aircraft types listed covers a range of operating cost of approximately 100:1, Danny might wish to first narrow down his mission profile.
The max-20 limit applies only to a G-reg (under UK CAA rules) and only if maintained under Private CofA rules. Under Transport CofA maintenance rules there is no such limit. In reality, a group of 20 or more will offer very poor access, albeit at obviously a relatively low cost. A further downside of such large groups, from what I've seen, is that it is so difficult to keep track of who has been hammering it down on the runway, and generally keep track of pilot competence and currency, etc, that the planes involved
tend to be old and in poor condition, which in turn raises the maintenance costs out of all proportion to the expected inverse of the group size.
Every other sharing gotcha must have been covered by now elsewhere
