Having learned to braze copper I can tell ya cleaning the inside is important and that a wire brush in a drill the appropriate size will certainly do the job well enough provided it's cleanish steel you're working with.
Through-holes look cool I guess but it's not going to be anything but a headache to fab. If you haven't done much fab work before I'd suggest using angle to build your trailer so you can get a better handle on making square cuts and making good 90° cuts and joints first, they are much easier than cutting mitres. Look at
existing trailers and other things made out of tubular steel (tables, chairs, etc.) to see how they are braced and joined.
If you're gonna braze this just use brass, there's no need to waste money on silver. Personally I use nickel where most people use silver but I'm cheap like that. If you're looking for an excuse to work with tubing specifically just get some 1" and 1.125" cr-mo and make a pile of small pseudo-frames.