Thanks for your input!It's not worth it. The iMac can be used as a monitor in a pinch, but I absolutely would not use it as your primary monitor. A decent 27" 2K monitor is $250. If you're spending upwards of 1200 or 1600 for the Mac Mini or Studio, include money in your budget for a decent display.
You've gotten your use out of the 10 year old iMac no doubt, see if you can send it off to a school if still works or safely recycle it if it doesn't.
I am happy with the system I have and really don't want to upgrade but am concerned with not being able to upgrade and security issues. Thanks for you input.Your Mac is within the time period where several Macs supported this feature. See here for additional information Turn Your Old iMac Into a Monitor Using Target Display Mode
I'm all in favor of new uses for older Macs but probably isn't a great idea. The older Mac isn't gping to generate a picture anywhere near what your new Mac is capable of generating.
I see RazOr beat me to it.
I am happy with the system I have and really don't want to upgrade but am concerned with not being able to upgrade and security issues. Thanks for you input.
Use your iMac as an external monitor...
I didn't know that.Luna Display ($120)
Luna Display — by Astropad
allows you to use iMacs that otherwise can't be used as an external display as one (unfortunately, it's priced close to what you would pay for a brand-new external display)
Also see:
Use your iMac as an external monitor
How to use your iMac as an external monitor