An aluminum sided trailer's framing is wood studs, every 16 or whatever inches.
A filon sided trailer's framing is just enough aluminum members to frame the openings, plus another piece here or there.
So you'd have to figure on screwing a wood frame into place inside your aluminum framing, IMO.
There is no debate about the order of installation. The top piece goes first, and you work your way down. Each piece has a groove at the bottom that receives the top of the next lower piece. You staple the siding to the wood frame, and the staples go on the bottom flange, which hangs down even lower than the groove I mentioned. So the staples are covered up by the next piece.
At the very top your stapes will be covered by a gutter trim strip that joins the roof and siding. At the bottom, you will "break" (bend) the bottom piece so that it wraps under the side, and then staple the siding on that underside lip.
For cutting windows and doors, I would imagine you'd use a router through the aluminum siding after it's installed, but I am not sure about that.
For ideas on sourcing the siding, give a call to Bontragers, they are pretty good people.
http://bontragers.com/Personally, if you attempt it, I think you will regret it. I probably would be easier to find a fiberglas guy to cut and patch or otherwise repair the existing siding.
