there has to be different "grades" of lexan because there are some office buildings around here that have lexan windows and have had them for years and years with no discoloration or melting issues. I know cuz my office building has them.
And while they are easier to scratch than glass, they aren't going to scratch all up just because of some "road dirt" the yard crew people fling all kinds of crap against the windows as they mow and edge and the only two scratches are where the actual weedeater string hit the window.
So just getting some plastic from home depot and trying to make a window out of that isn't a good idea. But getting the quality stuff, and installing it properly, you shouldn't have any problems.