that sounds perfect! I would build it however high you like, whatever is comfortable for you. Remember, take your time building it and make sure it will be easy to clean.
I have settled young homing pigeons (sqeakers, this is when they still make the squeaking noise like they did when begging for food) in ten days. this is when they are young with proper training. If you get some that are breeding age, and flew at their old home, they may go back. It all depends on how close it was, how much they flew, their age, etc. so go with young ones if at all possible.
As far as training, have a little trap door for the birds to come back in. Train them to use this by pushing them they it, and putting a small little cage around it. Also make sure they are the whole surrounding area around the loft. They will get spooked and you may lose them if they don't recognize the area. I typically do hard core training for a week, and then fly them. They should go thru the trap ten or more times without hesitation before you fly them.
Always fly them hungry, good is the number one determination for pigeins. have a food call, so that when you're ready for them to come in, you whistle, clap, etc... and then they will know it's food time and come back in.
Remember,,,, never fly a bird you can't bear to lose!