I'm a big HK fan and not at all a Larue fan, but my understanding is that the early OBR's are supposed to be some of the best. Even with my bias, I'm not sure I'd go the way you're thinking without being 100% sure.
At least Geissele has a cool rail for the MR762 now, and they have a trigger too...that together will run you about a grand probably but you should be set up at that point unless you need the G28 stock which is a few hundred more.
Or...you can go full-on BALLER and snatch this up:
https://www.gatewoodsupplyco.com/HK-417-165-Complete-Upper--AH-Date-coded_p_2009.html