I have 4K monitors that I am basically running at 1080 because I am having the OS do display doubling because my vision is aging. If your budget is tight then my recommendation would be to not spend extra for 4K if you can get a high quality 1080 monitor for the same money. Be careful of getting too big of a display… your vision has a maximum amount of the screen you can take in at once, so if you get one that is too big you’re likely going to have to sit further from it for general usage. I’d stick to around 27" to find the optimal price to size advantage. Brand wise, I have had good luck with Samsung, LG and ASUS, and I just choose which ever has the best price for the features I want at the time I go shopping.
I think it’s probably more important to decide on the technology you want in your monitor. I don’t think anyone is making OLED monitors yet (way too expensive) so you’re probably going to get a LCD monitor with some sort of LED back lighting. I choose IPS display technology because it seems to have the best viewing angles… but it’s more expensive than TN.
I would absolutely avoid any curved display. To me this is just a gimmick and the curve is going to act like a focus for any light behind you, and increase glare.
You can get an extra wide screen monitor, but be cognizant of how you use a computer. The web, for example, seems built for height and not width, and so much of the space on the sides is wasted. If you think you’ll want to run two things at once, side by side, then this may work great for you… but I’ve never personally be able to multitask like this and find I mostly run one app at a time and found a widescreen monitor to be a detraction. I’d rather have TWO cheaper monitors with extra height than one squat and wide one.