You can find some really good deals for the mount. I use one that is the "low profile" and it works great...however i did have to remove my rear sight to make it fit correctly. Also they say that my mount is for a G3 or MP5, but there is a little bit sticking up on the MP5 that makes it not fit by about 1.5mm. Not a big deal, I just use it on my G3.
So far as "best red dot" I don't know. Everyone has preferences and opinions but generally with airsoft, I find my cheapo BSA(brand) red dot gets the job done just fine. If I could go back, I would get a more squarish looking red dot because I think that those look cooler on the G3 but meh.