Thanks for sharing the pics, todmorg. Those three transformers on the top level I bet aren't lightweights. The flat bulkhead separating the top and lower compartments internally looks pretty beefy. And that looks like some really huge aluminum choke inside the machine near the front? Aside from that, nothing really jumped out at me, which is good. Looks pretty clean actually. I didn't see any microprocessor. Interesting that there are no surface mount electronics to be seen either (that's a good thing though actually... makes it more possible to "do it yourself service" in my book.)
If I had the machine, I'd consider routing my coolant lines through the machine, with cooler plumbed into the back. That would give the front a nice, uncluttered, clean look. That coolant flow sensor is not a bad feature! I've fried a couple expensive tig torch cables because in one case, someone shut off the water flow when I was welding. Here is a fried ck "superflex":
That was a very expensive mistake. (It stank pretty badly too.)
The dynasty 350 setup I used (at one of my customer's shops), has the cooler turn on automatically when you power up the welder. It was a pretty foolproof setup. I think that's good for a watercooler (to be as foolproof as possible), because all it takes is one welding event with it being forgotten to be turned on, and you can cause some expensive damage to your TIG torch leads. The flow swtich, if that's what that is inside the 250EXT, sounds like good insurance to me. Those hose clamps inside the machine look pretty good too!