Replacing deep well pump thus a few questions. What size pump to assure GREAT pressure for showers. Is submersible better? Tank is old, should I replace too & best recommended?

A submersible is best. It is easier to push water than it is to pull it.

Pump size is determined by well depth, size of casing and volume of water cell can produce.

Tank size is determined by demand and well production capabilities.

Pressure at the shower is determined by the pressure settings of the tank and pump switch cut on and off settings.