I'd go with Novak for the top end, right off the bat. They have a 1911 sized set of standard Novaks that looks and work great on the Hi Power slide.
I would definitely go with a C and S, A2 sear as the original factor sear can peen out after 9 or 10 thousand rounds. If you start getting hammer follow, it's usually the factory sear dying on you.
Since Hi-Powers went to cast frame, grips will often not quite fit tightly and they will often shift a bit on the frame, causing the grip screws to continually come loose. Bed the grips to the frame just like bedding a rifle action. BTW: since the HP uses Euro-Style screw slots, cut and grind a driver bit to fit your grip screws or go to Torx or Hex screws.
I personally prefer bobbing the standard hammer on the BHP as to using a rowel hammer. Also, rounding the back edges along the entire, exposed back portion of the hammer greatly reduces bite.