As far as I'm aware, all Synology NAS devices come with two Surveillance Station licences so that takes care of the NAS/NVR aspect of the problem if you're only having two cameras.
For cameras, you want something that fits in with your house finish and how you want to position them.
Field of View is a good starting point - 90 degrees obviously gives you a 50% view of the front of your property and the idea is to get the angle so you can see the face of anyone approaching. I'm a Dahua fan and my go-to option is always a 2MP Starlight as these don't need illumination from an IR source to function. If you are getting IR make sure you get a decent range, some of them are only rated for 10m while 20-30m gives you really strong foreground illumination. The main issue with IR is reflections on number plates which tends to make unreadable at night. There is a link in post 2150 for a decent camera that won't break the bank and is compatible with Synology Surveillance Station. In your scenario I would go with the 2.8mm lens at the rear and the 3.6mm lens at the front. I would also buy the Dahua mounting bracket that holds the camera horizontal, gives a good stable platform and a bit of space to loop away any excess cable.
I actually did an install at a property like yours using the Dahua IPD-HDBW4231F-E2 'Dolly Parton' camera. I ordered one camera with the 2.8mm lenses and one with the 3.6mm lenses and then I swapped one of each on each camera. One camera 'set' at the front of the house with the 2.8mm camera covering the whole front of the house and teh 3.6mm doing just the front door and gate. I did the same thing at the rear with the 2.8mm rotated to do the whole length of the garden and teh shed at the bottom and teh 3.6mm covering the garden gate. It worked really well with a Synology NVR-218 which has 4 Surveillance Station licences as standard. To do it on your system you'd need 2 more licences which are as expensive as a basic camera (about £50 each).
https://ipcamtalk.com/threads/review-dahua-ipc-hdbw4231f-e2-m-dual-starlight-cam.28860/
Hikvision also make excellent cameras and I'm sure you'd also be very happy with something from their range. I just don't have experience to suggest anything.
Mounting is always interesting and there is a huge amount of personal preference. Some people like bullet cameras but I find they are very prone to birds perching on them and also spiders love the warmth and the light, so they get busy straight away with the web building. Turret cameras are very popular because they avoid the birds issue but they still suffer a little bit from spiders. Dome cameras are completely immune to birds and spiders but if they are mounted badly they can suffer from raindrops and if you don't leave the little bag of silica in there they can also get foggy inside if they are poorly sealed when you refit the cover.