Sorry, but to replace an extremely old hw by very old hw doesnt make any sense to me.
The best strategy is to get a normal system of the last 1-3y new, that fits your requirements.
Nothing fancy, pure mainstream. Then you get all the good things and HW standards of today
in combination with CPUs which also will give to you a performance advantage on top.
The only question is whether you want to go Win7 or Win10.
Win8 and 8.1 is crap with the perverted Smartphone GUI.
Win7 will give you more privacy protection and stability, as there you can still parametrize Windows updates to update only with important / security updates which is one of the most important basements to gather stability.
Win10 is a data grabber and instantly moving/changing target where updates can not be customized anymore and bring instability by introducing too much changes. An application as a service makes sense. But not Windows as a service. From the Operating System I expect ultra stability. And exactly this you can not reach by introducing too many changes, which are out of the control of the administrator.
But if you choose a HW (mainboard, etc) which supports Win7 and if you run on Win7... then look closely to get also a good Internet Security Suite like Norton or Kaspersky, because your PC needs additional protection by such commercial tools. Especially because Microsoft has no interest into Win7 anymore and only the biggest security issues become fixed.
I personally will stay longer on Win7 for more privacy and robustness, even if it wont be supported by Microsoft anymore, because a mature Win7 along with a Security Suite should give me enough security with the things which I do.100% you wont reach anyway also not with the newest stuff. So go figure what best to take (stuff thats proven and works).
When looking for a CPU, you can look at passmark.com which CPUs give the best bang for the buck.
You should also choose a CPU where many PCIe 3.0 lanes come directly from the CPU.
Then with a good mainboard and a good layout you have more flexibility and performance for any
potential extension with add-on cards, shall this be required.
All that you should look for is for mainbords that support i.e. M.2, USB3, USB3.1 gen2
at least 1 PCIe 3.0 x16 sockets for 1 GPU if you are casual gamer.
An option if eventually to build up a system Xeon based with Supermicro mainboard.
Does not need to be newest platform / CPU. Simply something E3-XXXX CPU based.
There are plenty of options what to take, you simply need to choose wisely and carefully.
Win7 Prof, CubPro9.5, UFX+, XTC, ADI-2 Pro FS/DAC, RayDAT, ARC USB, Sonnet USB3-PRO-4PM-E