First off, if you're going to have 250 amp AC/DC then you also have the capability to have 250 amps AC/SC for SMAW (MMA)...
The next question is @ what duty cycle will the machine be capable of @ highest amperage? The same question should also be asked for the GMAW process and the Plasma cutter as well... What OCV will you be using?
Where will this machine be built? I could advise you in selecting the most appropriate components of both electrical/electronic and other non-consumable parts also so you don't end up designing a machine that breaks down in 30 days or less and instead last up to 20 years or so!!!
You should also design it for single and three phase power input because you will in effect increase your potential customer base.
Why? Because quite a few of the small (This also depends on your definition of small!) shops run on 3 phase power and the same could be said for some DIY"s as well!
I would also agree with Fixalinc in designing a machine with substantially more current output and I would say 400 amps @ a minimum of 60% duty cycle would be acceptable for purchase amongst a very large share of the market along with @ least a 5 year warranty also!
Okay! How about a 3 year warranty @ minimum...
I'll even real world test it out for you to find out if you designed a machine that will be very desirable to just about everyone looking to purchase you machine!
Respectfully,
Henry