Well, my advice would be since any newer Intel iMac would be a big step up from what you are using now, I would look at older iMacs that still can run OS X 10.6 Snow Leopard natively.
Look at 2010 to mid 2011 model iMacs. These were originally installed with OS X 10.6 Snow Leopard.
OS X 10,6 is the last OS X version to support older PowerPC software like Freehand MX.
The older aluminum design iMacs had more features for the money.
The new slimline iMacs, while cool to look at and are faster, other features have disappeared.
No longer a built-in optical drive.
No dedicated FireWire port.
No digital audio in port.
The SD card slot has been moved to the back of the iMac instead of staying on the right hand side panel of the iMac.
No Infared receiver for remote operation via an Apple remote.
The new 21 inch screen models can't be upgraded at all. They are glued up, sealed up, throw away computing appliances, now.
The new 21 inch screen models can only have a total of 16 GBs of RAM.
The 21 inch screen models, also, use slower 5400 RPM notebook style hard drives.
Tthe new models come with OS X 10.8 installed and you can't run older versions of OS X without using a virtualization program to run OS X 10,6 Snow Leopard Server so,you can still run your version of FreeHand.