First of all, welcome to the Forums! You will find a bunch of helpful folks here who will be able to answer your questions.
Belly skin aluminum is thinner than the side panels so it's less expensive. The best place IMO to get it is through an Aircraft Supply house like Aircraft Spruce
. There are lots of competitors, just do a search using "belly skin" or "aluminum" as keywords. You'll also find folks who have done this and can tell you the thickness and specs.
I do know, a '54 does not have clearcoat unless it was added at a later date. 1964
was the firsyt year it was offered as an option. As far as polishing goes, there are LOTS of options & opinions, again search on "polishing" to get some idea of whats involved to do it yourself or have it done. To have it done professionally, it'll run about $125-$150 per foot, so a Wanderer which is considered an 18' trailer would run about $2500 +
Again, welcome aboard...happy posting!