145 mm is your center-to-center measurement. Some folks call that their "inside" measurement.
Center-to-center is important for contoured and padded saddles. You want the center of the saddle "cheeks" or dome to be 145 mm apart, otherwise your bones are missing the carefully engineered sit bone landing spot designed into the saddle.
145 mm + 20-30mm is probably your outside-to-outside. This is important for padded and contoured saddles because you definitely do NOT want the saddle to be LESS than your outside, and you might want a cm or two extra on the saddle, depending on how active a rider you are. If you tend to sit down and not move even a smidgen, then the extra wiggle room doesn't matter. But if you shift to change position and center of gravity for corners and hills and to increase power to pass folks, you will want a little leeway. (you don't want your bones to miss the saddle!)
Your outside measurement is also vital for suspended leather saddles (Brooks, Velo Orange, Selle An Atomica, etc.) Your outside measurement must fit within the width of the saddle MINUS the left and right widths of the cantle plate. Otherwise you land your bones onto the metal rather than onto the suspended leather. (for example, in all likelihood you would hit metal on a Brooks B17, but would be fine and dandy on a B67, B66, B68, B18, or B72 or similar)
I do not find my Surly steel to be any less forgiving than any of my other 3 steel bikes. It is butted steel. Not sure what the LBS guy is talking about, but different people feel different things. (for example I can feel road buzz on aluminum bikes, other people don't) If YOU find your bike to be rigid, then get a B67 if you want a leather saddle.
32mm tires are quite wide enough to absorb a lot of bumps. Not sure why he thinks you need wider tires or a more padded softer saddle. I ride 23 and 25 mm tires on one of my steel bikes, and that is the bike with the unsprung B68 leather saddle. No problem.
Yes, it can take a couple weeks to break in a Brooks. They last 30 to 40 years if you remember to rub proof hide on them every few months.
www.wallbike.com gives 6 months free trial on all Brooks saddles. You mentioned interest in a Brooks in another thread. If you don't find the Specialized dealer gets you a Specialized or Terry saddle that works, I'd suggest trying a Brooks through Bill at Wallingford Bicycle Parts (aka "Wallbike")