Mark Housel from L&M Precision has a very good reputation and has done work on HKs. I had him do something else on a Glock, his work was great. Also check with Bruce from Primary Machine. His work is highly regarded but I don't know if he will do an HK.
As far as optics are concerned, for a defensive use I would not consider anything except for a Trijicon RMR. For general shooting and competition, Leupold and Vortex are options but I'd have some extra optics available to rotate when the primary optic needs go go back for repairs. Ho experience with holosun on pistols.