I used a GDR PT3 in college, here are my experiences:
1) In terms of build quality, B&S tubas are great. Take care of it with regular maintenance and do your best to avoid damage, and it will last a very long time. Then again, that's true of just about any tuba.
2) Not that I remember.
3) Nothing serious. I remember doing very little, if any, slide pulling. I really don't like pulling slides, and I don't remember anything problematic.
4) Bigger than you'd think, but it's still not really a large tuba. It's very much a 4/4 tuba (actually, it's 4/4 CC tuba with an F tuba bell). The tuba and its sound are both rather compact, but the bore is pretty decent so it will take a lot of air. No, it won't sound like a 6/4 tuba... but that's not what it's designed to do.
5) The one I used was 5 rotary. Can't comment on the 4P1R.
Overall, I think the PT3 is a fine tuba, but it might be a little smaller than what you want it to be. I also think it's unreasonable to expect that a tuba will last you from 10th grade through college, because you will develop tremendously as a player (and as a human!) during those years. I think the PT3 can do everything, but for teh big stuff it's going to be a lot of work, and it just isn't going to ever be a "big" tuba.
I am currently considering a PT20, which is in many ways similar to the PT3, but a larger and true 4/4 CC tuba. Hope that helps a little...