Catrin:
Steel soaks up vibration better than aluminum - so it shouldn't matter that there are no "isozone monostay thingys" in the frame. Steel won't need it.
You will probably be more hunched over (yet more upright than a road racer) on a touring bike. Keep this in mind that short distances won't feel funny - longer distances in this position are difficult for some. (hopefully you have no pre-existing back problems) Most touring is done with hands on top of the bars (holding onto brake hoods) and not in the drops. The drops are there to provide an alternate hand position on really, really long rides (and for downhills, etc.)
the 51cm 520 has a top tube of 21 inches and the 48cm a 20.5 inches. That is only a half inch difference. Seat Angle on 520 is 74degrees (51) and on a FX bike it is 75 (15inch) degrees (or 74.5 for the 17 inch or 74 for the 19 inch) The standover on the 520 51 vs. 48cm is 29.0 vs 28.1. You have a 7.6FXWSD or something similar right? The top tube is 21, 21.2, 21.8 for the 15,17,19 inch sizes on a 7.6 FXWSD. Some people feel more streched out on touring bikes. I would want to test ride the exact size I'd need or be put on something with comparable geometry to test ride. I don't know about riding the wrong size. It may work, who knows? Stranger things have happened.
Good luck in finding your next bike Catrin.
"Things look different from the seat of a bike carrying a sleeping bag with a cold beer tucked inside." ~Jim Malusa
2009 Trek 520-Brooks B-17 Special in Antique Brown
2010 Surly Long Haul Trucker-Brooks B-17 Standard in Black
1983 Fuji Espree Single Speed-Brooks B17 British Racing Green