Good question, but pause to think: What if all your wiring was in EMT metal conduit? (In which the conduit is the ground). There'd be no choice to bond the grounds, and thus, it is OK.
This question is even easier since you are working out of a sub-panel, which already has its ground bonded to the main panel; that is, one ground rod for the whole house. I have a facility where the 120/240v is served out of two main panels, each with its own supply transformer and its own ground. Circuits from both main panels go into a spiderweb of EMT, and while the neutrals never cross, the EMT certainly does, which means the ground does. What's more, the EMT is attached to an all steel building via girder clamps that are entirely made of metal. It would be impractical to separate the grounds. All this to say: If I am allowed to mix grounds, you certainly are.
The reason why Zinsco/compatible breakers are so gosh darn expensive is because Zinsco panels are obsolete and have a somewhat checkered field failure history -- while they do not suffer as badly from the issues that plague FPE's line of non-breakers, the Zinsco breaker line still has some of the same problems: namely miscalibration (i.e. failure to trip under low to moderate overload conditions) and breaker-to-busbar arcing/overheating damage.
As to a replacement type: the resistance of modern electrical grade aluminum alloy (AA-8000) to salt air corrosion (pitting) is unknown, but presumed to be poor -- also, salt air and current drive can aggravate galvanic corrosion of aluminum, weighing against the use of unplated aluminum busbars in salt air, and also against zinc-plated aluminum (the standard for aluminum busbar panels) due to bimetallic effects.
This leaves us with copper busbar panel options; some use various forms of plated busbars (either silver flashed or tin plated copper), while others use a bare copper busbar. While silvered contacts are generally considered to have excellent corrosion performance overall (tarnish is conductive enough in thin layers that it doesn't impact contact performance the way oxides do), the presence of chlorides (salt spray) can result in nonconductive silver chloride being present. On the other hand, tin plating withstands salt attack very well in salt spray tests, and so does bare copper.
Another factor is that if the existing enclosure is appropriately weatherproof and in good condition, an Eaton retrofit kit can be installed by an electrician to convert the existing panel to a type CH (copper bus BR is not available in the retrofit kits). Of course, if the enclosure's shot and needs replacing, then replace it.
Best Answer
I don't think there's a minimum height, but there's a maximum height for breaker handles.
So for a main breaker panel, where the main breaker is at the top. The center of the main breaker handle, cannot be higher than 6' 7".
Check with the local building department, as they may have a minimum height.
Practically speaking, you'll want it at a comfortable height to work in. You're not going to want to sit, or kneel on the floor while working in the panel.