As a make they are pretty good, a mate of mine has a factory full of the stuff with very few problems.
Spares and support is very good to the dealers but not really designed for the general public to access.
If you know somebody who can get you in then they will go as far as supplying parts with custom wiring looms and the like if the part you need is no longer available.
On an older machine like the kdo (I presume you won't be buying it from a dealer?) you will probably be better off repairing anything that breaks with "close enough" generic parts rather than paying migatronic's prices.
On the upside most welding repair places will be happy to work on it.
Much the same as any of the big name brands, they cost a lot of money new so (if the parts are still available) they are expensive to match.
Just because the welder is now only £300 because its 20yrs old a replacement power switch may be £300 just because it was originally a £3k machine.
Owner's manual for the KDO series machines here: https://www.migatronic.com/media/1815/50115300_manual_kdo.pdf
The "260" is the bottom of the range - so it is simple, with not a lot to go wrong. It has 12 power ranges, starting at 40 Amps. Weight 112 Kg.