If your goal is to be "as upright as possible" you will likely be happier buying a frame that fits you and having the shop switch out the stem for a taller one, rather than getting a frame that is too big for you and dropping the saddle.

(the "upright" feeling you like is the relationship between the saddle and the bars. If you raise the bars on a frame that fits, or if you drop the saddle on one that doesn't; in either case you are setting the relationship such that the bars are higher relative to the saddle. The problem comes along when you have a frame too big and you end up with a too-long reach. Too much distance between the bars and the saddle. Now you get all kinds of other problems. And the shop is right, moving the saddle forward to correct a too long reach on a too large frame is a recipe for misery.)