ABB can provide a keystring with ArcWare for just about any robot. I've seen a Lincoln interface on the teach pendant of a newer system that had a PowerWave R500 connected. It was connected thru ethernet to the PowerWave.
Just because you can doesn't mean you should 
ABB fully supports ArcLink\XT (XT signifies AL over Ethernet)
and they also support just about any welder out there as long as you can connect the welder through some kind of signal based interface (i.e. outputs / inputs and analog reference), be it discrete, DNet, EIP or whatever your flavor of fieldbus is.
I personally would be very reluctant to use a big robot like that for welding, while technically possible, the sheer mass (i.e. Inertia) of it is not suitable for arc welding.
Imagine a 3mm weave with 3hz cycle and the motion performance of that 
If I absolutely had to pick one of them I'd definitely pick the 6700 over the 6650.
But since you're dealing with refurbs I'm guessing these are "left over" automotive units, if so I'd probably just refurb them, sell em and use the earnings to buy a 2600 w. Arc Ware already on it.
You'll probably end up paying a couple of grand in software upgrades to get Arc on that 6700 and in the end you still have a robot that was never really mean to do what you're doing.
So - Just because you can doesn't mean you should
With all that said - if this is their tinker toy and your buddy plan on doing a bit of everything, MH, polishing, etc. and just the occasional arc welding job, then I'd say go for it.
