If you are talking about red dot sights, HK pistols in general do not lend themselves as well to that concept as do Glocks and M&Ps. This is because of the design of the slides; for one thing, the firing pin block is in the way, located as it is in the middle of the area between the breech face and the rear of the slide. It can be done (and has been done), but the resulting package is rather bulky and odd-looking. By comparison, RMRs, JPoints, DeltaPoints, etc., on Glocks and M&Ps, if done carefully, look quite symmetrical and proper.
IOW, for a range toy, which you seem to be interested in, it won't be too big of a deal for you to buy a rear sight mount and slap an RDS onto your HK. Its when you're talking about carrying an RDS-equipped pistol, especially concealed, that the bulk/aesthetics comes into play.
I have been playing with this concept for several years now. I have three Glock 19s with integrally-mounted red dot sights, and had a USP Compact .40 done. That was a disaster, mainly due to the afore-mentioned location of the FPB. The mechanic who milled my slide went a bit too far on that part, and the gun would not fire. In his defense, it was the first HK he tried this with, and there is always a learning curve. We settled up, no hard feelings. I hear that he has since "got it down" and has done several HKs for folks.
What's more important here is this… even if it had worked, the resulting package was too bulky for me to conceal, without radically changing my holster and other parameters… something I simply will not do, since the way I've been doing it for many years works quite well.
The issue with RDS pistols in general is this… it is expensive to get into, and once you're in, you feel compelled to go whole-hog (more expense)… and after you have done so, THAT is when you will discover that there is more- much more- to the concept than meets the eye. To be specific, if you get a carry pistol set up, you will find that it is marvelous for distance shots… and painfully slower on close-up shots. The reasons why are beyond the scope of this post, just rest assured that if you ever intend to try carrying a pistol with a red dot sight, the learning curve is not steep but it is long.
I know, you said "target shooting only". Trust me… once you see how far superior the RDS is to irons for deliberate sighted fire, you'll be tempted to try it on a carry gun. And that is a whole new ball game.