I personally have no experience of blackhead in turkeys, or chickens for that matter, but until we moved here 5 years ago, there had never been a chicken on the place!
The only problem I have had with my turkeys is firstly mycoplasma, transmitted by the large number of reared pheasants locally for shooting. Secondly, they do not like loud noises, and I often lose a couple a year when the RAF fly over in their fighters and helicopters. In terms of space per bird, I have never really measured it. I generally rear a couple of dozen a year, and they run alongside my chickens in a very large area between the breed pens that I have for my different breed chickens. I would guess that the area is approximately 15ft by 120ft. They do seem to enjoy scratching around, but I generally have to put them to bed every single night. Happily, I have managed to train my collies to do the job for me! Oddly, this does not appear to stress the birds at all, yet me picking them up and putting them to bed really annoys them!
Incidentally, I still maintain that Turkeys are the Sheep of the bird world - they are not particularly intelligent, and seem to think that there destiny in life is to die - normally about two weeks earlier and weighing less than you would like.
Weight wise - the hen birds come in lighter than the stags, but then again, it really depends on your customers at the end of the day. I don't generally get requests for more than 22lb, but I get a lot of requests for 10-12lb birds! Bear this in mind when choosing whether to go for white turkeys or bronzes, as the bronzes tend not to be as heavy. And yes, we slaughter on farm, as happily a good friend is a licensed slaughterman. My husband and I do all the plucking, dressing, final preparation etc....
Catherine.