I have a custom built one from a local company. It was worth every penny and I never had an issue with it. I just didn't want to make my own first PC.

That being said building your own will still be by far the cheapest way to do it.

PC Part Picker will give you a good idea what to look for. I'd go there first!

The bright side is FFXIV pretty much runs on anything so it gives you a fairly flexible budget to start with!