Barrel length depends on exactly what you want to do. There isn't much difference besides length and the velocity each length will push the round to.
If you want compact while sacrificing some of the velocity you'd get with a 20", then go 18". If you want maximum velocity potential and don't care about length, go 20".
As for threaded vs. unthreaded, again it depends on what you want. You said bench rifle, so going without threads would be fine. If you want it for 3-gun style competition or you want a flash hider or muzzle brake or want to eventually add a suppressor you'll need a threaded barrel for muzzle devices. You need to decide exactly what you want out of the rifle before picking barrel type/length.
As for having a BCG and charging handle for each upper, YES. Switching one BCG/CH between uppers is just silly. Besides that over time the BCG will break in to a certain barrel/upper receiver. Switching it around all the time isn't going to do it much good. Won't do damage or anything and it
can be done if you
really want to, but I would never have one BCG/CH for a multitude of uppers.
Just putting this out there, I see fluting listed as part of what you want on the bench gun. Bottom line: it does absolutely nothing besides maybe shave a tiny bit of weight, but if its a bench gun that shouldn't matter. If you just want fluting for looks, fine, but it's pretty useless in a practical sense. With that in mind I'll suggest BCMs mid-weight 410 stainless barrels.
Linky. (Click me). They have shorter lengths as well, including 18". I have 3, two 16" and one 20". All are
very accurate, clean easily, look great, and they're heavily profiled enough for anything besides super-precision use. Priced well for the quality.