Motion activation is easy, just about any DVR does that, and it is crazy simple stupid how it is done. The DVR or the Camera(both can do motion detection) will keep a number of video frames in memory and compare a couple in a series and if too many pixels change between images the change will trigger a motion event. Super simple, and super effective to the point you can have anything like a cars lights driving by set it off. False positives will happen, but it is better to have a surveillance system than not have one at all.
I happen to work with Geovision and Avermedia. Of the two, Geovision has more features but Avermedia is the one that is simpler to use. Depending on how far away the cameras will be and what type of detail you are after will depend on the type of camera. If you suspect people coming on your property then you will want vandal resistant cameras. If you go analog then you will want to go 600 to 700 TV lines. The type of lens you need will depend on what you want to see. A lens that is 3.8mm will have around a 1:1 aspect ratio the higher the mm lens the closer the view and tighter the angle, the smaller the mm lens the smaller the aspect ration but the wider the angle. If you want some serious detail then you will want to look into IP cameras or into HD. While the IP camera will have only a slightly better image when looking at it in the DVR Software, the true beauty of the IP camera comes when you go to use digital zoom. The IP camera is a Megapixel camera, and the higher the Megapixel then higher the amount of information it will captuer, the higher the amount of information you capture then the closer you can zoom in on objects before the image will become pixilated. A 1.3Megapixel camera is comparable to a 700tvl camera, the 1.3 megapixel camera is a little better. However, when you go to IP cameras, because of the increase of recorded information per video still, your cost of storage space will dramatically increase. I believe a 1.3 or 2 mp camera will average 3+ gig/day. That number really is pulled out of a hat also, the amount of space really depends on how much motion the camera will see, the type of codec used for encoding, the FPS the camera records at, the MP of the camera. I have seen systems with IP cameras on them go through 6tb in a day or two.
If you go PC or standalone on the DVR will depend on how the alarm events are done (such as a door opening). For products such as Geovision you will need to get additional cards such as the GV-NET I/O and the GV I/O 12 IN card. If you go standalone, many have the RS-485 connections already there for the alarm events.
This is by far the most impotent piece of advice I am going to tell you:
YOU GET WHAT YOU PAY FOR
If you buy cheap, you get cheap.
Man must shape his tools lest they shape him. -- Arthur R. Miller