I have twin 80amp barrel fuses for the main, what is the max amp to switch to a breaker box?

There is actually no way we can determine that. It has to be power company determined.

Service upgrade costs vary and what is your responsibility, Upgrading to 150-200A is one option. 200 A if you have hot tubs or all electric stuff.

As an example some locals say that you have to pick up part or all of the tab if wiring is underground and almost none of the tab from the pole to the service drop if the feed is overhead.

Where you responsibility ends depends on locality too. It may be at the meter or the service drop. In almost all cases the power company furnishes the meter and you have to use "approved meter sockets" for your locality.