As for your filter, it most likely will be a lot of flow. I am running one on my 20 long but it's far from 20 gallons with hardscape. I ordered a 2217 impeller when it was on my 29 as my impeller wore irregularly and now it's crazy. The length makes it work. You could either throttle it back, or put a T fitting in the tank. The T fitting will not only add hard angles but it will also basically half the flow in each direction. The only reason I would think this will work is I needed a temporary fix on another tank and did this and it reduced the "pressure" or "current" like crazy, not sure how to describe it but it wasn't shooting out water and calmed it a lot. Not sure if it effected actual flow much. It also makes a very tidy looking outflow if you aren't going with anything special like lily pipes. I would probably go with stainless lily pipes (and possibly polish them) on your tank if you are doing something special.
I also think it would be cool if you found something chrome for your lighting fixture since you are making it. Knowing you are into motorcycles, I am thinking like a chrome cover of some sort. I just think it would bee cool with the theme and probably look "vintage" without too much work if you found something the right size.
I look forward to seeing this expand.