With budget in mind, bang for buck, a good Seneca II with -EB engines and the big tanks (123 USG).
The ones I've looked at and have seen had the best payload with full fuel of the lot, were relatively cheap to buy and will do basically the same a III or IV does for a lot less money.
Watch out for the Engine type, the -EB engines have 1800 hrs TBO, the -E only 1400....