3D Text elements have their own requirement. On desktops, that means at least 1GB of vRAM (I would suggest at least 2). I don't know how that translates for laptops (most of which use "shared" video RAM with regular RAM). 16GB RAM is probably a good minimum. With USB3, you can save money by planning for external SSD "work" drives and standard storage drives as opposed to an internal SSD.
It simply comes down to: what do you want to be able to do... for the next four years. Factor in: how much things have changed in the LAST four years. A brand new Mac sold today with sufficient resources might be expected to last up to 6-8 years (keeping up with software updates). I think people using the top of the line 2011 machines are still hanging in, but the next release of macOS (and whatever version of FCPX that goes with it) may leave them behind... It's just that new and improved software always needs faster and more (minimum) resources.
FCPX 10.4 is very nice. It should be serviceable without upgrading for a relatively long time to come [my oldest "living" mac is 17 years old now]. If you outfit whatever Mac you buy for today's FCPX, you could simply use the minimum requirements as your "shopping" guideline.
I love my Macs (I've bought 8 of them over the past 30+ years [approximately 1 every four years on average...]). Macs are pricey... Love hurts 😉