I have an older iMac (2009) and wonder if I purchase a Mac Mini can I connect it to use the IMac display, as it is a nice 24 inch display?

Nope' your Early 2009 24" iMac misses the Target Display Mode requirements by a half a year and 3 inches.

You need a Late 2009 27" iMac to use in Target Display Mode.

see > Use your iMac as a display with Target Display Mode - Apple Support

Yes, one of the reasons I decided to stop buying iMacs and start buying minis. I have 2 iMacs I haven't been able to sell - they have incredibly nice displays but are too old to trust on the Internet for people who aren't savvy. (They are old enough they aren't receiving security updates and the browsers that run on them aren't either.) When I retire the mini I'll be able to use the display with the new one.

Early 2009 iMacs (24") cannot be used directly as a display.

The only way is via remote network interface. I know. I have

one sitting around collecting dust.