57amps is about normal for LRA (lock rotor amps) for a high head pressure hot compressor, it only lasts for a few seconds or less
no help with a new start kit,, your compressor is SHOT
yes you can install any RV A/C brand you want (they all fit the 14" hole), just be aware of any difference in controls or thermostat options
roof air with built in controls and NO wall thermostat are the easiest to replace
if yours usess a wall thermostat, get something that works with that control
or get the new control when you get the new A/C, be sure the wiring in the wall has enough wires/connections to work with the new control