The unit, these are not exact, but will be fine for you if you add 1" to all on the welder.
255 LX or 250 EX = 9" wide, 21" deep, 17" high. A guess, 50 lbs, so build for 70 lbs to be safe.
On the tank. I would make a 10" base plate, then 150-330 you are covered. 9" will probably be fine.
EDIT: 150s are 7.5", also 330 tanks weigh over 135 lbs and are 9" dia. I recommend bigger bottles to save on refills. 150 is easy to move around, bigger ones, your Dad will need to roll and lean them here and there. But worth it in the long run.