THE FIRST thing to do is sit down and think out THE MISSION you wish to accomplish with your bus.
Is it a weekend warrior? Weekends and occasional longer (how long) trips? Full time live aboard and travel? Prefer warm and dry, the humid south, winter wonderland adventures, RV hookups or boondocking, or?
What your mission is determines all manner of things for your build.
How much insulation you need, how much electrical power, propane, comfort appliances, etc., are all dependent upon your intended mission.
A weekend warrior, particularly one you plan to use RV parks exclusively, will have much lower requirements than something you plan to live in full time or want to boondock with.
Those lower wall panels are not all that difficult to remove, even if they're riveted, and getting the old insulation out of the walls is a good move, especially if you then spray foam it. Spray foam will seal, bond things together (fewer rattles and drumming), and give superior insulation.
As for the earlier recomendation for the Harbor Freight metal shears... I bought a set and they're great for cutting the 16ga metal we're using to blank out windows, hatches, vents, etc.
I also have 3 or 4 Harbor Freight angle grinders (the ones that go on sale for $9.99 all the time). That keeps me from having to change from sanding disk to metal cutting blade to grinder.
Their aim chisel/hammer is also used a lot. I bought a set of round headed bits for it that have helped with hammering out a few dents and the chisel is great for cutting rivet heads off.
Their air riveter is also in my inventory along with a small bench grinder and a supply of their grinding wheels and sanding flap disks for the angle grinders.
Check YouTube and you'll find a myriad of videos on demo of the interior. We have a set of time lapse videos on you youtube channel (same name as here) showing removal of the ceiling and side walls (ours were screws). Check our ours or someone else's before you decide to leave your lower walls and build over them.
YouTube: HAMSkoolie WEB: HAMSkoolie.com
We've done so much, for so long, with so little, we now do the impossible, overnight, with nothing. US Marines -- 6531, 3521. . . .Ret ASE brakes & elect. Ret (auto and aviation mech). Extra Class HAM, NAUI/PADI OpenWater diver