Hi!
Here is another suggestion. Try to find an early 2011 iMac. It can hold an additional hard drive, for example an SSD, along with the Superdrive. Install OSX 10.6.8 on the regular hard drive and your newer favourite system on the SSD. Or the other way round. Why 10.6.8? Because there is a lot of software that does not run on later systems. I know it works because I have two of these, one 21″ and one 27″. They are very robust machines and have not failed in 8 years. As a bonus they have a Thunderbolt (1) interface in addition to the older FireWire.
Best wishes, Helge