it depends on your car, how hard it will be to take out. But you can get the same size alternator you have now so it will drop right in place of your old one.
You have to up grade the wiring of your "BIG 3". Meaning, make all the wires connecting your battery to your alternator a higher gauage (usually the same size as your power wire) You will probably want to upgrade your ground on your battery to a big wire also.
As for the most power you can run on 200amps, im not positive but 1800 watts should be able to be ran on that.