Obedience training is wonderful!
Just remember that no dog is perfect, and there will be an adjustment period when you bring the dog home. Be prepared to work through any little problem there may be. Sometimes even housetrained dogs may have a few little accidents when they arrive.
Take what the dog was bred for into account, and know that hounds may be noisy, greyhounds cannot be let off leash, and border collies have sky high energy and drive, etc etc.
For a first dog, here are few that sound good.
Windsor
Chaunce
Cosmo
Joey
I know you said "medium sized" but please also consider a rescued greyhound. They are clean and quiet in the house and are couch potatoes!