For the 8 indoor cameras you can either us Body cameras or dome cameras. For the 4 outside cameras i would recommend IR Dome cameras as you will need the Infra-red to see at night (start from around £80).
You also need to consider that as you have a drop down ceiling, can the cameras be reach by a person to disable? If they are reachable then I highly recommend that you get a Vandal-proof camera.
If your looking for very high quality cameras then go for cameras with Korean chipsets such as the Serage Range
You will be using 12 cameras in total, your best bet would be to purchase a 16 Channel DVR as in the future if you wish to purchase further cameras then you just need to buy the cameras. 16 Channel DVRs start from around £260 and can go up to £700 depends on the quality, hard drive size and features.
Installing yourself isn't too bad, just make sure you have the correct cabling and connectors and you should be fine.
All the Best