Personally if it were my first tank kit in years I'd probably just build her straight from the box and concentrate on just getting everything straight and the paint and weathering right. As Tojo said it can be built into a really sharp looking model straight outta the box.
I wouldn't worry too much about the tracks, as you say the tracks are 'live' without any sag so the rubber band effect will look fine with them. You can pick up a turned metal gun barrel, which are nice, they save you from all the careful sanding but I'm not sure you need to add one.
If you really want to add some PE you might pick up a set of Eduard US headlight guards. They look much better than the thick plastic parts and are fairly cheap and easy to apply. PE periscope guards also are an improvement but they're a PITA (at least for me to apply).
http://store.spruebrothers.com/product_p/edutp028.htm
One of the biggest and simpest things for a Sherman is to add some storage. If you look at any photos you'll see piles of knapsacks, rolled-up tarps, cardboard ration boxes, etc strapped or heaped up on the vehicle. You can buy these, make them yourself, or if you know any armour modelers they probably have a box of storage goodies from old Tamiya kits.