I have had your dilemma - and probably still do... but decided to stick with my 7.5tonne. I have hired a couple of 3.5 trucks but they don't have the storage space of my larger truck and don't feel as robust. I looked at changing to a newer wagon - TBH I don't think a 4 year newer box will make...
https://www.facebook.com/teamhayler/?locale=en_GB
There is a nice looking horse advertised on the Team Hayler facebook page. (they are based in Yarm Yorkshire)
Did you ask the previous owner if you could view the vet record? You seem to suggest that you went straight to the previous vet? If you didn't ask the previous owner first they may well have said no for no other reason than they thought it was discourteous that you didn't approach them first -...
Start at the height you are comfortable with. Din't feel pressure to move up. The jump off will usually be higher than the round itself. When you are feeling confident at this level try the next class.