As it has been touched previously, the concept is to dress in layers to better manage moisture and to regulate tempreture. A basic layering system would be:

-Baselayer top. Any synthetic top will work and can be found cheap in places like Target, Goodwill etc.
-Insulation layer. A fleece, down or synthetic jacket.
-Wind protection. A simple windshirt (Golite, Marmot etc).
-Outter shell. This protects against the rain.

For your lower body then a pair of thermal bottoms or my favourite, Capilene bottoms and hiking pants or shorts is just fine for most conditions-- even down into the lower 20s. I use my MH Mesa hiking in all conditions except for the winter.

I would leave the sweatpants at home. You would be better with your thermal pants and hiking pants for the lower end of your temp range. I would avoid those fancy softshell jackets-- they are made for winter use and are heavy, bulky and overkill for your 3-season needs. I would also avoid "all-in-one" solutions-- you will eventually either become too hot or too cold-- I believe that it is better to have two tops-- one lightweight and one midweight, for example a Capilene 1 and a Capilene 3. Again, the idea is layers and versitility.