I have an old 486 that I go online with, check e-mail, write papers with, etc.
If those are your Dad's only needs then this system is more than sufficient.
As stated, the monitor issue is nothing. They're all compatible as long as it's not a Mac monitor.
I'd try looking for preowned machines, but beware some of the moron sellers. They'll post things like "Paid $1,500 asking $800" for some piece of crap that is a decade old.