Welcome to the site, I always want to say theres a great bunch of guys here LOL but thats not the case. There's gals too. So I guess I will have to try to remember to say great folks!!.
For cutting the openings,
Displace the contents of the inside with water 100% and start cutting. I use a cheapo grinder with a metabo cut off wheel so that if the grinder dies from water damage its only 14 99 down the drain. A couple guys here have had great results with the metal cut skill saws. Some use the torch and others use a plasma.
Maybe check out the builds like eman said and look at several different designs. I like the ones that have two large doors and the middle section between the two large opens also.
Don't get going to fast, plan the grates well, your stuck with them unless you remodel. In a recent build of which I can't remember the builders name, has a center support that allows him to remove the upper shelves completely to fit a whole animal on the main grate.
I would consider leaving the tank as it sits and remove and plug the bungs on the top, leaving the lifting eyes in place to make it easy to move later. It won't be balanced after the fire box but you can rig it to balance no matter.
The Lang's and such most all have a giant firebox so I would go how ever wide the tank is and make it square. Maybe a hinged lid on the fire with a grate underneath for maybe the occasional grilling need. If not a grill grate then at least a warmer plate to heat water or simmer beans and such.
Reverse Flow or tuning plates for sure.
As far as the trailer goes, its unlimited.
I don't care for rain and hot sun, so mine has a roof 14x7'9 and 36"x14' wings on both sides. Two stainless tables that fold out and down to have more floor space to work. My remodel should start no latter then next weekend provided I can get my new to me trailer home this sunday.
In my signature line at the bottom of the post has links to some of my stuff. My trailer build isn't the greatest but it was what I had to work with. The plan from the beginning was to add an axle or switch over to an larger tandem trailer. With a little prep I will lift the roof, smoker, and tables as one unit over to the new trailer.