If I buy a used CNC machine, how much support can I realistically get from the manufacturer and the distributor? The manufacturer is only interested in the latest models; the distributor only wants to sell new machines.
For any particular machine, that is a good question to ask the service department of that company before you buy the machine. I think for the most part you will treated well. They will be selling you parts as long you own the machine, right? It makes sense that they would want to help you keep that machine going. And what difference does it make whether you are the original owner? If the company sells a machine in 1995, they make profit off of that machine sale in 1995. I have heard of some companies charging for annual support, but I don't think it matters if you are the original owner.
Machines of certain ages will be easier or harder to get parts for, and controls may or may not be current or supportable. This is especially true with a dealer that may not have always carried the product and may not have long term technical capabilities (for older units).
While a dealer or manufacturer may have made a profit on the original sale, if ongoing support is included in the initial deal, the manufacturer is not likely going to walk another company through the learning curve without suitable payment (service contract, hourly rate, etc.).
You need to decide if the value you are getting is going to offset the potential additional costs the current owner does not have (or is not willing to spend for), and how this stacks up against the price of new/current technology.
Do not expect to get a bunch of free training, but most equipment suppliers are happy to sell you training time.
I do not know what kind of CNC you are looking for, but be aware that PTP machine controls and software have changed faster and more fundamentally than have the controls and software for heavy routers. A ten-year-old PTP is probably running a controller and software that bears no resemblance to its modern counterpart. A 10-year-old heavy router, on the other hand, is probably going to have a fairly standard machine tool control that looks and works a lot like new controls of the same brand. What this means is that unless your PTP supplier has an "old timer" in tech support, the old PTP may be as big a mystery to your tech support as it is to you. More standard industrial controls, like you would find on milling machines and routers, are usually supported by the control manufacturers for a very long time after the last new unit is sold. For example, a Fanuc 6M control from the early 80's is not only still supported by Fanuc, but by a number of after market vendors as well.