I’m not a red dot expert but I have two, a HiMark and a UTG. Both do the job and are rugged enough but are on the low-end on cost - about $50 for the UTG and $120 for the HiMark 3 years ago. I like the UTG better because it’s also green dot which is much more visible in daylight, and it has variable brightness.
IMHO replacing the feed ramp and hammer bushing is a must, but if you’re going to open it up, you might as well do all the internal upgrades at one time or you’ll find yourself opening it up again when you break down and get the rest. While taking it apart isn’t difficult, it’s not a 5 minute process - you don’t do it unless you really need to.