Don't buy a pre-built they are usually more expensive to buy with worse specs then custom ones.
Personally i would get a custom with these specs:
GPU - Palit GTX 1050 Ti 4gb
CPU - Ryzen 5 3600X
RAM - 32gb/16gb (either one will be fine)
Motherboard - ASUS ROG STRIX B450-F AM4
SSD - Kingston A400 480gb
Power supply - Corsair CV550
Case - which ever one you want really.
wont cost much either around £700 for a good pc like this