In my opinion, best bet is to buy as close to home as possible. Ideally, the seller should be someone willing to show you the particulars of the machine as each has it's oddities (they're often modified by handy farmers). In a perfect world, you could call upon this ideal seller if you have any questions about maintenance or repairs further down the line. At the very least have them switch the thing on in your presence so you know it works before you hand over the cash. Noise and exhaust are things to keep in mind, depending on how confined your parlor/intended parlor is. Goats in particular are sensitive to loud noise. Used stainless milk jugs, pots, etc. are good buys if you can find them, as they don't degrade much in quality over time and are super expensive new. Any plastics (hoses, inflations, etc.) are best bought new as they need replacing every season or two anyhow. As far as being interchangeable between cows and goats, there are a couple issues you might run into. Many pumps have a fixed pulsation ratio (milking phase/rest phase). You'll need an adjustable one in order to milk both cows and goats. You'll also need a slightly larger (and thus pricier) machine to milk cows with, if I'm not mistaken (I only have personal experience with goats). The claw apparatus for milking cows is also much more expensive than the inflation/shell setup for goat milking.