The Mk3 was a thoroughly weedy little thing - constant thrust variable noise - particularly when flow with full tip tanks. Mk 5 was vastly better - a fair bit more oomph, much better canopy and cabin pressurisation.
We did 300 KIAS 250 ft low level navexes in the things which were a complete waste of time. It was almost impossible to regain time without exceeding max continuous thrust and it drank fuel. Plus it was much heavier on the controls and bounced around like a pea on a drum. But at 240 KIAS it was very pleasant indeed...
With decent lightweight avionics, an ex-Finningley Mk 5 with tip tanks and no concrete leading edges, moustache spoilers or other aerodynamic add-ons would be the one to go for if A to B cruising is your thing. But for performance, a Mk 4 would be best (samethrust as a Mk 5, but without the extra weight or pressurisation bleed) - except that they're probably all knackered by now. Although the aerobatic purists always said that the Mk 3 actually handled slightly better.
The -A models with VOR/DME aren't really worth it for civil use - you'd be better off with something like a Garmin 430 where the non-A models had the Eureka 7 control box.