Frankly, I'm not crazy about the sound of the solid state "tube emulator circuit" in my R600, but then I don't like many bass distortion or overdrive pedals either, so maybe my opinion isn't the best to listen to. In any event, I never use it. And' I've never heard a Series I with a real tube, so I can't compare.
As far as the tone of the whole amp less the tube emulator, I would imagine they're pretty doggone similar whether it's a Series I, II, or III.
You do definitely need to find out the impedance (ohms) of the cab, so you know what's going to happen when you plug it in and run bridged mono or normal mode, or if you ever add another cab. Usually it's printed on the nameplate (if there is one). It is quite easy for anyone with a soldering iron and a screwdriver to rewire a 4-speaker enclosure for series or parallel, though, so you want to make sure. I've never seen one, but I have to imagine that someone has invented an ohmmeter that has a 1/4" phone plug as its probe for determining the impedance of a speaker cabinet. Though if he's the original owner and says it hasn't been rewired, I suppose you could trust that.
I hope this works out for you -- a dollar a watt for a complete rig is a darned good deal!