You want a hardtop or convertible?
In order of cost the TR6 is gonna be the most expensive., then (arguably) the GT-6, followed by the MG. The running costs are going to be the same order as the initial cost $ wise.
To get a nice GT-6 you'll end up dropping a decent chunk of change. Buying a poor one will end up getting expensive fast. TR-6s are even more than the GT-6s (I have one, and it is not cheap to run), especially if you want to make it nice.
Why do you want a TR? What's wrong with the B that you are looking to change it?