Ah, someone who knows where I got my username!!
I get asked about that windshield all the time. The windscreen is a Maier windscreen from JCWhitney (see link). It is the Type 4 (square bottom fits on top of my rectangular headlight), clear, 7/8 in. bar type, $69.99. I broke one of the plastic mounts where it attaches to the windscreen, and could not find any replacement parts, so I made some from $2 of sprinkler head parts (really) which now hold it together.
An alternative windshield that 2 co-workers have is the National Spitfire which seems to have a more durable mouting system to the windshield than the Maier. Slipstream might also make an equivalent. I don't have any pics on how it mounts (I sold the bike), but it has bars from the lexan screen that go to 7/8 in. clamps that mount on the handlebars. The mounting is similar to handgards. It took a lot of fiddling, and the wing nuts that held it in place always came loose until I used some real screws.
The windshield doesn't stop all the wind, just kept some off my chest. I really don't know if it is worth it, but it does look cool. It will give you some headshake in crosswinds or from large trucks on the highway. With new D606's on the front on my XR650L, it could shake at 65-70 mph, about 5mph lower than without it. A coworker with an XR650L has the same problem. I put a TKC80 up front, and could go 80+ without headshake, but still large semis will make the front move around. I'm just telling you what I experienced. In spite of this, I have it on almost all the time, and I have ridden the bike on several 400+ mile days. My $0.02.
I used 14/45 gearing, meaning I put an XR600 front sprocket and kept the stock rear one. It worked well for me. The 14T countersprocket was enough to let the bike launch from idle, and it still was not too buzzy on the highway. If anyone runs dirt on an XR650L, get a 14T.
Regarding your "just buy an L and ride it" comments, you are correct, yes, absolutely!! I put 10k miles on and had a good time. I later found out that wimpy old me couldn't ride it in the sand, so I sold it and now have a green DRZ-S. A different horse for my new riding expectations, actually. The bikes are different, but I miss the long-distance touring capabilities of the XR650L.
You're right, if one buys a used bike, one can ride it for a few years and then sell it for only a small loss. That XR650L and I had some good rides together...