I recently purchased (on this forum) a modular sprinter. Here is the build link :
https://sprinter-source.com/forums/showthread.php?t=44522
It's unit strut, not L track. At first I didn't like the strut and considered swapping it out for L track, but after having it a month, I've really come to appreciate the strut and the infinite possibilities it offers. Currently their is a cabinet attached with the strut that houses the solar panel. Last weekend I attached a prototype bedframe- will remove it tomorrow (should take a few minutes). I strapped all sorts of stuff we were hauling to walls and attached some d rings to the floor to hold down heavy items. For my next road trip I'm going to attach some shelving to the strut to store stuff on. I'm not real handy, so will eventually have someone build me a simple removable galley. In the meantime, I'm quite happy strapping my igloo water spigot to the wall when I go camping.
You'll find quite a few posts about securing seats to the floor with L track. To sum them up briefly, some people do it, but there are real concerns about liability/insurance issues in the event of an accident where someone gets injured.