I believe the blackhead problem is only a problem when a) other poultry have it first and b) when the birds are in close confinement or intensively raised. I may be wrong, but for several years I had turkeys, ducks, chickens and geese all together in a field without any problems.
I'd suggest getting poults at at least a week old: that'll weed out the weakest before you invest in them and they'll be just a bit more hardy. They'll need to be on heat for about further 3 weeks still after that and for a good while beyond that, you need to ensure they don't do anything too daft like standing out in the rain before they are fully feathered. They really aren't very clever about getting themselves somewhere sheltered, even as adults!
Lack of trees is probably a good thing: it'll save you having to get them down from the branches when they decide to "go natural" and roost in them!
I very much enjoyed my turkeys: good luck!