Hey… Welcome to the forum…
Before I answer your question specifically, I would like to point out that LE is in beta and has many known performance related issues and instability… High density mobs and particle effects are two confirmed examples of this.
The current state of the game means that right now, existing hardware that should be capable of better performance, struggles in specific instances.
Once the game is more stable from a feature & content perspective (so that they are not optimising things that will be changed/removed), and EHG can dedicate time & resources to dealing with the performance aspects of the game, its likely that a lot of us playing on ok hardware will experience generally better performance overall.
With that in mind, I think that any system you put together specifically for gaming with current hardware (i.e. something from the last 12-24 months) will be more than adequate to play LE when it releases as version 1.0.
I would not buy a bespoke system from one of the system integrators - I would build the machine myself.
The best approach if I were upgrading my own system, would be to look at it from a budget perspective rather than wholly a performance standpoint.
Personally I dont think you could go wrong with either Intel or AMD on the CPU side but for a GPU I would go for Nvidia 30 series right now. Make sure that you pair the CPU & GPU well for the resolution you want to game at… i.e. dont get a CPU that cannot keep up with your GPU and vice versa…
Obviously cooler & memory to match the CPU and I would suggest 32gb Ram (16 is fine but you may as well futureproof a little).
Definitely get an NVME harddrive for the speed - thats a no brainer and you will never go back to a mechanical drive (or even SSD) after using one…
Keep in mind the monitor you use… if you get/have a 60hz monitor @ 1080p, it can only show 60fps and you dont need a high end GPU to keep 1080p pixels happy… If you want a 4k 120hz monitor and want to play games at that res, then you need to get a GPU to match…
Wish I had the budget to upgrade my old i5-7500/1060 setup… 