I have a 20g long frag tank. Not drilled and only uses HOB filtration. It's a great little frag tank, but it's a little more work than I figured. It is an SPS tank so there's that to consider.
Not having it connected to a display means double the testing, double the water changes, double tracking the ATO container, double just about everything. It adds up pretty fast, so keep that in mind. It's nothing major... and some, like me, even get a little enjoyment out of the extra "work". This is a hobby after all, and just staring at a tank with nothing to do to it isn't very hobby like... right?
If you're going to use it house frags you're eventually going to put in your display I'd highly suggest that you get a light that is comparable to you display. This helps with the transition and help frags maintain color after being moved. For instance, the PC Rainbow in my frag tank was a dull rust red color for months in my frag tank... after a month in my display tank it's showing green, red, and even blue on the tips!
(I have a super old Chinese black box over my frag tank and a AI Prime on my display)