James
Welcome to the site!
Others have given their opinion as to whether you should get an
America or Speedmaster

, so I am not going to add my biased opinion here.
However I can answer the other questions - It is not legal to use anything but "approved" pipes on the roads. That said I have "off road" pipes on my
America and have ridden past loads of cops without any trouble. The dealer who sold them said that I would have to refit the stock pipes for the MOT, but another member here, Grzegorz, got his
America through one with the off road pipes on!
Re leaving out in the rain, if you do this, (like any bike) its soon gonna look like sh!t. An (ex) friend of mine cost me about £500 by leaving a bike outside for 12 months that he told me he would keep inside. In fact, this 3 year old bike was so bad, the dealer didn't want to take it in part ex for my
America!! At the very least, I would get a cover for it, if only to make it less obvious to any thieving scum that may be around. you have only got to look in this weeks MCN to see their latest tricks.
Good luck with your choice, you will enjoy either bike (especially an
America)