The difference between a truss and rafter design is the truss will span a distance with the exterior walls supporting the load. Your walls will be 16 feet apart, so your span is 16 feet. You can make the trusses wider to include an overhang, but that 16 foot width is what counts in truss design.
Rafters rely on a load bearing wall in the middle of your span. Sometimes on really wide buildings, you can have multiple load bearing walls. You don't have this in a small shed like yours.
For a short span, the truss is pretty simple. You can order them from a supplier and have them certified for that span, or you can just build your own. Again, for such a short span, it's pretty hard to get it wrong.
Why those dimensions? Is that all the space that you have?
For your purlins, use 16 foot lengths and overlap the seams. Use 4x6's for your entry door posts. I like my trusses to be 4 feet apart with 2x4's on the flat for my purlins. If you go 6 feet on center, then you need to put your purlins on edge. The wider you go, the bigger the lumber that you will need.
How close are you to Muellers? I like them for my metal siding and roofing. Good people with a quality product. Don't buy your metal from the Home Stores. It's thin and the finish isn't very good.
The secret to fastening down a metal roof is to tighten the screws just sung enough to bulge the washers. If you tighten them too much, you will ruin the rubber gasket and it will leak.
If you live in an area with high humidity, you should insulate the ceiling. Otherwise, it will create condensation. This will give you the affect of rain inside the building on certain days. If money is tight, you can avoid this, but don't be suprised to see water on your floor or what's in there on perfectly dry days.
Make the door as big as possible. Mine is 10ft by 10ft, which I consider just about right. It's a roll up door, which I also really like. No wind issues, it's water proof and opens real easy without taking up allot of ceiling space that I can use for my ceiling lights.
Try to pour concrete for the floor as soon as possible. If you put it off, it might never get done. Once you start filling up the building, it gets harder to pour concrete and easier to keep putting it off.
Eddie