1. Your Like is highly appreciated
2. Yes, you can add additional HIKVISION cameras to 8350, not only 8003
3. No idea, other standard cameras you can record with Synology, just try it with HIK protocol. Or try RTSP port 554 as alternative. I could test for you 8003 with Synology in about a week.
I think that original NVR from HiLook NVR-104MH-C/4P PoE for appx. €55 (picture below) or HIKVISION 7604 generally cheaper and better. Especially when you need PoE and 4K or more than 2 cameras.
4. You can buy
hereDS-KH6320-TE1 for apps. €150. They deliver to EU.