Well, it sounds like you're in wayyy over your head. But everyone has to start somewhere. I don't have much experience with the HK45 but I did used to own a USP45c. Based off what I know from that, and about polymer framed handguns in general, you're going to be looking at an injection molded frame with either an overmolded or pinned in place locking block and slide rails. Most other FCG parts will be pinned in place. Small thin components like trigger bar, slide catch, etc... will generally be stamped and heat treated sheet metal. Larger components like the hammer are probably EDM'd or milled.
You could probably get models made of most of the internals with a pair of calipers and sweat equity, although you would ideally have a sample size of several to over 100 guns from different batches to have some type of tolerance study if you're at all interested in production. Otherwise if you're just interested in a one off, its gonna take a lot of hand fitting.
As for the frame, for a one off 3d printing will be your best option. The frame is likely something like PA6GF which is printable on consumer grade machines, albeit with some difficulty (although I recall the frame of my USP feeling stiffer and harder than most other poylmer guns I own. Maybe a higher glass fill percentage than usual?). But this is going to be nowhere near the durability and longevity of an injection molded frame. But you're gonna paying tens of thousands of dollars or even hundreds of thousands to have the mold machined. Not to mention you clearly don't really know anything about this stuff so you'll also have extremely expensive engineering cost going into mold design. Speaking of which, a 3d scanner would be a good way to go for capturing the compound curves found in the frame. That will generally create a surface, it'll be up to you to generate a parametric model from that, which can be time consuming and difficult.
All this to say, if you can't design a pistol from the ground up, odds are reverse engineering is also way outside your skill set. But good luck nonetheless.
