I would say that it is best to get the sturdiest machine you can get. As far as stick welders go, I would try and get good sturdy machine with 80% or more duty cycle at around 120 amps, which is also able to to DC welding for 7018 and Ni-rod, since you are going to be doing cast iron (the buzzbox won't do this). For the wire-feed side of things, make sure the duty cycle is good, and make sure has a gas input, unlike the FCAW only horrible freight wire-feeders, so that you can do mig or even dual shield. Trust me, you want to be able to run dual shield when you get there, it is awesome.
As far as getting two machines or one, I would just go with whatever option is cheaper/available. I would make it a point to stay away from the 210 mp's, though. I have had many hours working with them and the wire-feed mechanism is really fiddly and likes to make rat's nests, especially with softer flux core wire. They also have a really annoying digital display that is meant to make the setup simpler, but I find it annoying. I usually wouldn't say this, but go for the miller if you decide to get the miltiprocess machine.
Also, have you looked at any of the cheap tig welders? I have seen some really cool stuff done with AHP and Everlast machines, which will do both stick and TIG. An AHP and a halfway decent wire-feed machine would give you all three processes for pretty cheap.