I can't help with the build/buy option as I don't know your skill level.
I tried once many years ago to make one in HO and it looked horrible.
Got the timber sizes all wrong and your eye just new it wasn't right.
I can help though with the HO/O scale question. The answer? S
Not trying to be funny, one of the premiere bridge kit manufacturers is Hunterline.
They recommend S scale bridges for On30. O scale would be too mainline in size and HO is too small, S scale is just right.
Hmmmm, seem to remember some story about something similar to do with porridge, anyway have a good day and good luck.
I'll need to build a few bridges as well and don't look forward to it after my HO fiasco.
I bought a Hunterline Queen Post S-Scale bridge two years ago and still haven't tackled it.