Using A iMac late 2009 as a Monitor

+2, Nope not going to happen the MM is way too new to support Target Display Mode. Please carefully read Target Display Mode. Please buy yourself an external display for the MM, they start at <$100 and can very easily be found on Amazon, local PC Store, Costco, Best Buy (assuming you are in the USA) etc...

Not possible. Newer Macs do not support Target Display Mode.

Not possible. The new Mac Mini does not support older Macs running in Target Display Mode.

Using an iMac as a monitor - Apple Community

FYI Target Disk Mode ≠ Target Display Mode

Your welcome.