When looking at properties, or designing barns, fence, what have you: I would center the facility around safety and efficiency. Will you stall your horses at night? Stalls connected to paddocks or will you lead your mares to them? Or do you want to keep mares out on pasture? Keep clumsy foals, and star-crossed mares/stallions in mind when planning out your pastures and paddocks. If your mares foal outdoors, what sort of predators are around? Know your neighborhood, keep in mind every cat, dog, mare, and stud. If you choose paddocks disconnected from stalls, will you halter break your foals to be able to easily lead them too and fro? Some foals, my filly included, will wander off from mom, even if the snow is a foot or two deep.
Lots of things to keep in mind. By helping out at other barns and breeders, doing the grunt work will do lots to help you plan out your facility by judging how efficient their system works. Note turnout techniques, feed, and methods of grazing, (rotational, etc,) and go from there. Do they properly manage their fields? Can they sustain horses on their property with minimal hay? Can you finish chores with time to spare for hands-on time with the mares? You can also look at what they are producing. I know people who breed four mares yearly, only to keep them for two years, bearly halter break them, and sell the buggers at auction for under $200. They don't take advantage of the miles of trail riding, and neglect to find their niche in the market. The ponies that they have could be shown young in the local pleasure curcit, broke as trail horses, and they might even break even with all their hardwork. Warmbloods are a different horse entirely, and have a different market than stock breeds, but there is no reason to be loosing that much money when you could easily break even with a bit of something left over. Try and plant yourself somewhere that Warmbloods can be marketed locally, as well as any easy place to ship. Note some big shows, and try to stay close to them so you have a place to market your animals.
Take my advice with a grain of salt, but these are just things I would want to thoroughly think over once I'm in the position that they are realistic. As far as horses go, I would recommend buying something of a package deal. Mare and foal, with mare bred back, as others suggested. You have a foal on the ground to see what she produces first-hand, and already a foal for next year. I would get something 15-19 years old, and make sure either she has a good record, or produces something real special. I wouldn't keep any stud prospects early in the game. If you build up a broodmare band, expect to be switching studs or purchasing more every few years to rotate new blood, especially if you are retaining daughters. If you had the facility and equipment, AI would be a good method to use, considering that opens up lots of studs, and then you don't need to keep your own.
Again, just my thoughts on the matter. Good luck, and make sure to do your homework.