I haven't used my 390XP for milling, most recommend the 395XP for milling, over the 390XP, but I sure won't say that it would be a BAD choice.
Just my own Best Wild Ass Guess, but I would say that it would likely be fine for a 30" Mill, Maybe a 36".
Remember, with Mill "Sizes" the listed "Size" is the length of the Mill Frame rails, NOT the mill's Capacity, OR, the bar length needed.
My 36" Granberg at Max capacity has 33.5" between the clamp faces, so regardless of listed size, figure 2.5" less capacity
On my bars, I don't clamp the nose end any closer than about 2.5" from the bar tip, you don't want to crush your nose sprocket or bearings, you will have 1" of bar clamp, and about 2" of Dawgs, unless you remove them, so whatever mill "Size" you buy, figure at least 6" longer on the bar, than the size of the mill, to fully utilize your mill.
Obviously Hardwoods are going to demand more from a saw than softwoods, and you haven't said what kind or size of wood you want to mill, so I can't say whether the 390XP will be adequate for your needs or not, but as has already been said, you don't need to have any concerns buying from Dan, he is a Well Known and RESPECTED seller here on AS, Many have bought from him, and I have yet to hear of any complaints, but a LOT of Thank You's, for a Great sale.
Doug