Sounds good, but you won't need the heat lamp and it will dry your tank out. Thermostat is a must, good call on that one (most new owners won't buy one initially)! What is the climate like where you live and where are you from? UK, maybe? If so humidity shouldn't be an issue so long as you mist down the substrate a couple of times a week of fashion a cover for the tank aside from just a screen top.

A 55 gallon will be a little big but won't be bad so long as it is full of good hiding places.

As for the snake being new, I would take it once it has been received, look it over to make sure that it is healthy and free of mites, and then put it into the enclosure and leave it be for a week or more. At that point (at least a week later) I would attempt to feed it, and if it eats I'd wait two days before handling it. If you handle it before it is settled and eating it may be harder to get it eating, so wait to handle until it has eaten at least once.

Switching to frozen - some take frozed right away, some never will. I would be more worried about switching to rats than F/T (frozen/thawed) and then switch to F/T after the switch to rats.

Welcome to the forum and welcom the addiction of ball pythons! I look forward to pictures fo your new one and also to sharing with you on the forum!