Can my old iMac be used as Monitor for my Mac mini?

Nope'

If you decide on a Mac mini, you will also need an external Display.

see > Set up your Mac mini for current Mac users - Apple Support

No.

Even if your iMac supports Target Display Mode, a new Mini would not support using ANY iMac as a Target Display.

When you said "Power Mac" – were you really referring to a PowerPC-based Mac released before 2006? If so, it's long past time for a new main machine.