Dnk, your current pc is really useless. I would keep HDD for external storage only.
I can tell you that configuration of new pc is always linked to the purpose of use.
If you do not play games, working with a lot of spread sheets, or making videos, you can use 8Gb of ram as today standard.
Built pc is cheap, because they use the cheapest component. Especially power supply unit are generic ones without any extra safety/efectivity features.
The best solution is to pick all components up separately from proven producers and ask a friend to build it.
Of course, you can buy cheap pre built pc but bear on mind in this case that maintain the "user friendliness" (speed of system, updates, hardware and software compatibility, stability) could be more expensive in a long run