I use a 1080p 144hz 1ms monitor. I think max res should be 1440p for performance and visuals balance.
If the Harrier sparks your interest then go for it. It's great for A2G missions and is versatile. Hovering is the most tricky part - like flying a helicopter. It's pretty easy to start and get the weapons going. Search for Chuck's Guide for it if you take the plunge, will be helpful.
The other jets are more tricky to learn due to their system complexity, but the next easiest is F16 - I enjoy this one also. F14 has a lot to it and is interesting to fly, but considered the best module in DCS - can do multi-crew. F18 is a jack of all, master of none. Viggen is great if you like sinking ships but can be complicated.
I enjoy the Harrier a lot and have many, many hours in it. Always fun!
I hope to see a Tornado GR.1 in DCS also - fingers crossed!
Hope this helps,