You asked about a dot. Depends on what type of shooting you are going to do, but in general, I have found that the best dot for price/performance is the Ultradot. The cheaper reflex-style sights tend to not have very defined dots; they are fuzzy or they won't hold zero or they won't have enough adjustment. The Ultradots can be slide-mounted on a 1911 and stand up to years of shooting and recoil. You can find them for around $100 used. Above that, the aimpoint dots are nice.
I would recommend a 4MOA dot for 50-yards and under shooting, and a 2MOA dot for over 50 yards. Note: the cheaper dots often appear much larger than a comparable high-quality dot even though they advertise '3MOA', etc.