Custom, buying pre-built usually involves having a cheaper MB or low quality PSU which can cause issues later on. Plus with the money saved on building custom you will be able to have better quality parts. Also building custom IMO is fun and is a great learning experience.
There's plenty of guides/videos online that can help you with the building aspect of it and sites such as PCPartpicker are great to help you find compatible parts/lowest price. We can also help you get a part list for the best price/performance.