The issue with instructor continuity is a real bone.
Are you consistent with the days you fly and book in advance or do you call at the last minute. People who plan tend to get better continuity. Instructors usually have standard duty days and finding one that matches the patterns you want to fly will increase the chance of continuity.
The other issue is that instructors get paid *all and will hop off at the first opportunity for a decent paying job in the airlines so no-one hangs around for long. The schools have little control over this.
Few people get through the whole course without at least one change of instructor. On the other side few instructors with 1000 hours will have got more than one or two people all the way through.
As for the age of the plane, when you learn to ride a horse do you want an old calm hack or a frisky 1 year old. Don't you want the plane to be more experienced than you?