If you're getting a pre-buit PC u are already losing around 20-30% of what you could get cause companies take a percentage just for building it. It's really not difficult, you can find many tutorials on the internet that shows u step by step everything you need to do.

Edit: also about the monitor, I'd recommend going with AOC, they're great for the price, probably the best choice if you want to spend on PC as much as possible.