Here is the spec page for that PC
https://support.hp.com/us-en/product/hp-pro-3500-microtower-pc/5270849/document/c03364089
Here is the general service manual
http://h10032.www1.hp.com/ctg/Manual/c03550748.pdf
The hard drive appears to be any 3.5" SATA 6.0 Gb/s up to 2TB.
Graphics is Integrated in the CPU, but there is also a PCI Express x16 slot. Check the manual and power supply for possible upgrades.
I'm not an HP employee.
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