Hi Brent,
I see your problem with the price of Milestone.
My humble suggestion is to register the free version, "XProtect Go".
It will only allow you 5 days of recordings. The main reasons though are that it is very stable, has its own mobile client, works with most IP and analogue camera and its free!
With regard to the cameras, i just started using " Sunell" cameras and i found that there were alot more bang for the buck compared to the big names that i was using.
As Dan said, 1 by 1 and you will get there. This will also help you to monitor cpu usage, and storage requirements on a per new camera basis.
Lastly, I would say that I would mount on the side of your garage closest to your door. For this, I wont use anything bigger than a 4mm lens though. From the picture, the door looks no more than 4metres away. My general rule of thumb, for every mm on the lens, cut 1m on ground. So if you put a 12mm lems, chances are you are focusing on an area 12metres away!
I hope this helps.
If you would like more info, you may PM me. I am new here but i've been integrating systems for the past 9years and I dont mind sharing what I've learned along the way.