A 20k bulb will have a great color, but no growth.
I wouldn't go that far. In fact, some people in the community are now only recommending 20k MH
bulbs coupled with 03
fluorescents as that provides "better" growth that other spectrums or other bulb combinations.
I used to run a 20K bulb from Hamilton as well with my two 55w 03 PC
's. Had good growth on just about everything. Recently I switched to a Coral Vue 14K and it is doing fine as well.
what about just using the fixture, and purchasing the bulb?
No problem with that, however, you may not be able to find one that will fit on or over your tank like you would like it to so you will probably have to do some modifications. If they had some pendant fixtures, that would be fine, but I have never seen those at HD