You could do fine with a 27 inch iMac, which I what am using. My 2012 model is still working smoothly, and I bet you can pick one up very affordable second hand.
This model has two Thunderbolt 2 ports which carry also displayport/DVI signals, as well as firewire. You could hook up a second display, as I did, using a Mini Displayport to almost any other connector (DVI, Displayplort or even HDMI) and use the second port to attach firewire, with another adapter cable. If you need more firewire ports, I am quite certain there are hubs, splitters or whatever. USB should be the least of your problems, the iMac has 4 ports, and you could always hook up a cheap powered hub.
If you want to use both of your monitors, you could look at the Mac Mini, which has almost the same connecters. You can attach two monitors by using a Mini Displayport to Dual Link DVI adapter. This saves the second Thunderbolt port for your Firewire.
My setup is this:
27"iMac (3.0GHz i7, 16GB RAM, Fusion drive - which is a small SSD combined with a large HDD) with one Thunderbolt/displayport connected to an external hard drive, daisychained to my second display (a Dell 27"monitor). My DM is connected to the second Thunderbolt connection in the iMac, with a Firewire 400 to Thunderbolt 2 adapter cable. Various MIDI-devices are connected to USB-ports, either directly or through a MIDI-USB-adapter.
Hope this helps. Any Mac user here feel free add your experience.
Marcel