Will this be on the floor, or up on legs? Just curious because if it is on the floor, 2x4s seem like overkill. Mine is 6'3" by 6'3", with 1x4 frame and 1/2" plywood deck. I trimmed it with oak, using a tongue in groove method of my own design, along with screws and glue. It is rigid as heck, and has been carried in and out the last 16 years, not to mention 2 truck rides and a stay in storage when we were moving. I considered modular, but it was my first attempt at building something like this, and I was afraid that might be more complicated than I wanted it to be. I will admit, it does seem to gain weight every year.
I never thought to treat the track, but it hangs in the garage 10 months a year, and I live in New Jersey, yet I haven't had any weather related trouble. I did have mice one year, and the area directly below their camp suffered some rust on the side of the track, I assume from urine. All in all, I went simple and it has performed very well.