This is sort of tricky since there are so many cameras on the market today. However, some guidelines.
-look through this thread of Network cameras.
-buy cameras that use the ONVIF standard. Easiest way to connect them to NCS. Especially if you want PTZ.
-even if it can be tempting do not buy the cheapest chinese stuff. Sometimes it works great, but quite often it causes a lot of pain.
-when you have found something that you like do a search on the Internet for that camera. There is so much valuable information there.
-what resolution do you really need? Megapixel cameras are very reasonable in price today, but they do need a computer with a good CPU. 3 cameras with 2 megapixel each (1080p) use an i5 processor. Probably works with 3 and 4 MP also, but don’t use the computer for something else.
-If you want a very reliable and stable system use a dedicated computer and have all the cameras on LAN and not wifi. If you have a good signal strength between router and camera wifi can be ok.