Oscar, the standard tank is 15 gallons, Imperial of course. Aiglet wings are 2' each shorter bringing your wingspan down to 32' with the added bonus of wing tanks. Unfortunately they also give you a longer (but only a little bit) take off run.
£3.5k is a bit more than the value of the core engine (usually around £3k) but if it has good compressions and no shiny bits in the oil filter it could be a goody. As a sales point, pitch up with a new oil filter and remove the old one, cut it open (not with a saw) and inspect it yourself for shiny bits
If the crank is good then you can (under the PFA) make it an engine to last your lifetime for £5k or so.
Cheers treadigraph, nice to know there is somebody else out there