Is it still possible to do a clean install W-7 on a used PC?

Yes, of course it is possible.
Do you have a license key?
Does it have win7 on it currently?
Is it the pc listed in your system specs, or a different one?

If I understand correctly what you've said so far:
You have a HP Compaq Elite 8300 Convertible Minitower PC like the one I linked you to.
You've installed a new hard disk drive(HDD) in it.
You're a computer novice.

If you do it the same way that I do it, you will need these 3 things:
The Windows 7 SP1 64-bit install disc
The files for the primary drivers(chipset, USB 3.0, audio, ethernet, graphics) for that PC
Simplix Update Pack 20.9.10

If you're able to boot from the install disc, and the keyboard and mouse is functional, you do a clean install of Windows 7 SP1 64-bit.
After that's done, you run the update pack - which will restart the computer multiple times and will bring the operating system up-to-date to September 2020.
After that's done, you install the primary drivers in the same order that I listed, and you restart the computer after installing each driver.

Do you have a wired keyboard and mouse, and do they have the round PS/2 connector or the flat rectangular USB connector?

I'm glad to hear your wired keyboard and mouse each have a round PS/2 connector.
Even though your PC has a USB 3.0 controller, the keyboard and mouse should both function okay when connected to their PS/2 ports.

If your wired keyboard and mouse were the type that has a rectangular USB port, I fear they may not function after you booted from the install disc.
That's because Windows 7 SP1 doesn't have built-in support for USB 3.0.
It requires integrating USB 3.0 drivers into it.