“Someone who is just getting started out in this great sport, what essentials should they pack if weather changes? Sweatshirt? Good tight fitting fleece?”
Never a sweatshirt! Never anything with any cotton. Check your clothing tags and make sure none of them contain cotton when riding. You want a thin thermal base, a mid-layer based on weather and a good outer layer.
I like a good tight base thermal and these can be bought anywhere. Some things to look for is cut and the silicone line. Some brands have a silicone line on the bottom of the shirt that help keep the shirt from riding up. Some brands are cut better and just flow and feel better.
I discuss mid-layers below and all the major brands make good outer layers.
“How heavy should mid layers be and how important is a base layer if you have a heavy mid layer”
The base is always important. It sits right on the skin and wicks sweat away, Sweat gets cold quick! Always run a base. Mid layers can get tricky. Fleece, wool or thicker looser thermal all work great. What thickness and type really depends on you, riding style and weather. I commonly don’t even wear a mid-layer until the ride home, but see others with a thicker fleece jacket on all day. Everybody is different. For years I used a super thin fleece I picked up at a big box store, but have yet to see it for sale again. Last year I wore a performance fleece and it worked just as well. I only switched to the performance fleece because it had my logo on it. I wore both my fleeces on one real cold trail ride last year—it was about as thick as a normal fleece. I would buy a performance fleece of some sort and see how that works as a mid-$15 to $75 usually.