Don't overthink this, it's assembled from small parts, so it can be disassembled by reversing the assembly process. I've built AR's part by part (believe me there's a lot of parts), with a few tools it's pretty easy. Specifically, you mentioned the buttstock and forearm, a fixed A2 buttstock can be replaced with a flathead screwdriver. If you don't know what a part is that I mention, just google it, the whole process isn't too difficult.
Buttstock: Remove the buffer spring (it's big you can't miss it, open up the rifle and remove the big spring in the buttstock). Use a flathead screwdriver to unscrew (counterclockwise) the top screw in the butt of the stock. Beware though, as a small spring and detent will come out if you're not careful (it's held in place by the stock you're removing). It'll screw right out, and now screw in the collapsable tube same way, securing it with the castle nut (make sure to put in the little spring and detent first). If the stock isn't on the tube already, simply slide it on, pulling down hard on the length adjuster of the stock so it'll slip over the tube.
Rail: First replace the original handguards. To do this, brace the rifle, and pull down firmly on the slip-ring (at the base of the handguards (towards you when the rifle is shouldered) while simultaneously pulling out on one half of the handguards. This takes a little practice and technique, but after some practice you can do this in a few seconds (if you *must* there is a tool to make it easier: handguard removal tool), then repeat the process with the other half.
There's two types of rails, drop-in and free-float. Given your knowledge of the weapons platform, I'd recommend the drop-in unless you want to have a gunsmith install free-floats (or you're incredibly daring and wish to attempt it yourself). To install drop-in rails...simply reverse the process above: pull down firmly on the slip-ring and insert in the rail system one half at a time.
Ta-da! You now have customized your AR. They're like erector sets, they're fun to tinker with and trade parts out on, so buy one without hesitation, follow my instructions, and enjoy.
Put together many an AR, and have shot them for years.