I don't know a lot of the answers to everything you asked, but the topic is worth a discussion
The molding machines I've read about are rated in "ounces per shot", which is the weight of material it will inject per cycle. So I guess you would need to know quite accurately what the volume of your part is going to be, to match it to a particular size machine.
The molder has its own press platens. One side is fixed, and the other side moves on the 4 guide rods you referred to. All you have to do is mount your mold on the fixed platen, then close the moving platen on it, and fasten the upper half of the mold to the slider. Then, its ready to shoot.
The finish inside your mold is exactly what you'll get on your part. A mirror finish is required for something like a lens cover. You need a good, modern high speed machine to generate a good enough surface directly off the mill, with no hand polishing. I've never done one of these, but I suspect I guy might still need to polish a little bit here and there.
The parting line needs to be in a natural location where the part can be ejected from the cavity without catching or hanging up on something. It is best along an edge, where it visually won't be so noticeable, I suppose.
I believe you'd want to shoot the injection in an out-of-the-way position that will not be noticeable, not right in the middle of the top face. The sprues still have to be cut off.