Can I use my older iMac strictly as a monitor

No sorry, only the Late 2009 27" iMac plus a few other later models supported Target Display Mode.

The list of supported iMac's is at > Use your iMac as a display with target display mode - Apple Support

Early 2009 iMac will definitely not work as a second display.

I own one and I know.

With that said, there is a way if you are willing to totally

gut the iMac, cut holes in the case, and take a chance on

some "hobbyists" graphics adapter card on eBay (which as

anything on eBay is a gamble) that is an

exact match to the LG panel in the early 2009 iMac and

possibly end up with a pile of scrap with a lot of money and

time spent...

Better off just donating it or recycle it.

Doubtful. There are a lot of hoops:

Use your iMac as a display with target display mode - Apple Support