The best source of information is Steve Camp's hipowersandhandguns.com. Mr. Camp is very experinced and articulate and covers all aspects of the BHP and several other handguns. He also provides data and pictures to back up his opinions. I think his website is outstanding.
I have owned 3 Hi Powers, all MKIIIs, one polished blue with adjustable sights and walnut grips, and two with the black paint finish, plastic grips and fixed sights. I didn't care for the 'anti-aircraft' tall adjustable sights (made by Millett I believe). The liked the fixed sights with vertical white bars which were set to point of aim for 115 grain and low profile. The black plastic grips were more comfortable for me than the checkered walnut ones.
The Mec-Gar mags work fine and I used them almost exclusively. I had the 15rounders and the extended 20 rounders and all were reliable. The 17 rounders with the big plastic base that CDNN sells (sold?) also were reliable but less well constructed than the Mec-Gars. They wouldn't take all 17 rounds when new but would one 'broken in'.
I sold all of my BHPs because the grip was small for me (I have enormous hands) and thought the safety was 'mushy' and could be wiped off inadvertently. The guns fed pretty much everything fine and were decently accurate (not as accurate as my SIGs) but they didn't fit a need for me. A Bar-Sto barrel might improve that. I wouldn't feel bad to have a BHP if my SIGs vanished but it wouldn't be my first choice in handguns. Rent or borrow one and see if you like it.
I've seen conflicting opinions on using +P and +P+ ammo in MkIIIs. I almost never use 9mm ammo that is standard velocity and felt like the MKIIIs wouldn't hold up to a steady diet of the hot stuff although it worked fine in the small quantities that I put through them. The pre-MKIII models have inferior metalurgy and I have seen surplus examples that looked beaten to death probably from a diet of SMG ammo.
Novak's and Don Williams at The Action Works build BHPs that are works of art. Both 'smiths require $$$ and time.
If I wanted a single action 9mm I'd get a 1911 (blasphemy, I know) or a CZ-75, probably in stainless. The overall top score in my LFI class was done with a stock CZ-75.
In sum, a BHP is a fine handgun but there may be better choices. Rent them all and see what works for you.
Good luck in your search.