I know this thread is a couple of months old, however, if anyone comes across it...
The horse in my profile pic was my (grulla, bleached from summer sun in the desert) Spanish Mustang gelding - purebred and registered. My parents raised them. He was an excellent horse, great for riding, working cattle (he was used on a cattle ranch for around 10 years before the owner had a heart attack) and I used him as a teenager for barrel racing. They are pretty universal horses and good for about anything short of large draft horse work.
The BLM mustangs you see up for adoption are not the same, most of them have a lot of other stuff mixed in as Bunny said, draft, quarter horse, thoroughbred - basically whatever was turned out with them, and that happens quite a bit with the market in horses crashing. You see tobiano patterned horses in there, which is not registerable with the Spanish Mustang Registry. Even my parents who raised them for well over 30 years have scaled way back ( I think they have 10 or 12 now, as opposed to well over 30 horses and several of those are in their mid to late 20's), and they have lots of land. The problem really is with the market being down, they are hard to sell, and horses eat a lot! So unless you have expendable income, it's not a worthwhile venture. I'm pretty sure that even my parents spent more in feed than they ever made, but it was a hobby for them.
Personally, I would not be breeding horses in this economy unless it was purely for something I wanted to keep. I see horses, broken to ride and gentle for less than $1000 in my area. These are purebred horses with papers, so a mixbreed isn't going to fetch that and by the time they are old enough to be weaned, they've probably cost you at least half that much in the extra feed for the mare and vetting. If you are going to raise it up to an age to be ridden and trained, you'll have far more invested just in feed than you will get for price. A lot of mix bred horses go through the auction for slaughter every year - yes even with horse slaughter being done away with in the US, there are still killer buyers that ship horses to Mexico and Canada with the same end result for the horse.
Foals won't be born true "white" - lethal white overo causes the foal to die fairly soon after birth. Most of the time their intestines are not correctly developed, neurological problems. It is akin to "peanut" rabbits, born with 2 copies of the dwarf gene - LWO foals are born with 2 copies of a mutated gene (frame overo) - same principle, they don't survive. Generally it would be born any other color and then gradually turn gray, and eventually look white. They'll still have dark pigmented skin underneath.
The Irish Draught horses I just looked at from their website are grays, not actually white. They weren't born the way they look now. Considering that purebred Irish Draught horses do not come in Sabino, you'd have to cross on a Sabino colored stud of another breed, and then it would be hit or miss on how much white is expressed. Gray, however, you have a 50/50 chance of getting if one of the parents is gray.