The spacing of your trusses depends on the anticipated load on the roof and the construction of the truss. Up here in snow country, the only difference between a truss for 4' spacing and one for 8' spacing is the species of wood in the bottom chord and the size of the gussets or truss plates holding the thing together. If you get a truss designed for 8' spacing by someone in your area, it should work just fine.
There is a cost difference in the trusses and every time I've designed a building, things worked out pretty close to the same price whether I used the 8' trusses at a higher cost each or the 4' ones but more of them. Personally, I prefer the 4' spacing and laying the purlins flat just for the ease of moving around on the roof while putting it together and because guys without a lot of experience find it easier to get the nail in a 3 1/2" width than a 1 1/2" width when putting the roof sheathing or metal on.
Our local code calls for double 2 x 12 top plates made of yellow pine which is stonger than spruce, pine, fir or "whitewood" as it's called in the big box stores. They should be attached to the posts with 1/2" carriage bolts, and don't have a splice on both sides of the same post. Use 4 through bolts per post. Down there in Alabama where you don't need to design for a possible 18 or 24 inches of wet snow, you can probably get along just fine with a single 2 x 12 top plate -- check your local building codes to be sure.
If you're like here, the code doesn't need to be followed for an agricultural building and you won't need a permit or have to get it inspected. Still, it gives you a guide as to what kind of strength you should design into your building for your area, so check up on it.
There are two methods in use here for the side purlins -- 2x4 on 24" centers or 2x6 on 36" centers. We also call for at least 18" of treated tongue and groove skirting at the bottom, 18" diameter pads set 36" deep under each post, "Y" bracing from truss to post on anything 12' tall or over, angled 2x4's connecting the top of the the end truss internally back into the structure at least 12 feet, preferably among the "V" members but sometimes also along the bottom chord, and some kind of center bracing connecting the bottoms of all the trusses such as a 2x4 stringer running the length of the building.
I live in a county with a pretty strict building code. Someone living just a mile from me but over the county line would have a lot more latitude about how to do things, but I'd prefer to have a building made to our code in case of a serious storm, either summer or winter.
