Preferably at least a semi-modern i5, 8-16GB RAM, and at least an nVidia GTX 1660.
Desktop VERY STRONGLY ADVISED, as laptops have a ton of drawbacks for livestreaming.
The NVENC core on a 1650 Super (NOT the non-super or Ti), 1660, 2060, 2070, or 2080 is on par with software x264 Slow, so will handle the heavy lifting of realtime video encoding, and look great. You only need a 20-series if you plan to game on the system at some point in the future, but the 1660 will give you access to SKUs with more VRAM (try to get an 8GB model if you can, but a 6GB model will work).
The CPU will still need to deal with decoding the incoming video stream from the ATEM, and any other sources like Browser Source alerts, video playback for media clips, etc.
NVENC can definitely do 1080p (it can encode two 4K60 streams in realtime, so one 1080p30 stream is no sweat).
3500kbps is going to be low even for 30fps (framerate also alters bitrate needed) no matter WHAT encoder you use, but should be able to get away with low-motion stuff without too many problems.
6000kbps for 1080p30 is the 'average' baseline, and 12000kbps for 1080p60.
Lower motion (card games, slow pans) can get away with less, high-motion (first/third person shooters) will need more.
