Is this a project you chose or is part of the project is to figure out how to accurately dimention all the parts?
You could get accurate measurements of each of the parts if you apply your trigonometry. Everything in ISOMETRIC view is 45x45x45 degrees.
Could you upload a few pics?
Look at each part. Start with the simplest ones, the ones that will be easiest to build, like the screws and nuts. (Unless your instructor requires it, don't make the threads on the screws and nuts. Just use simple cylinders for the screw bodies and the holes in the nuts.) Think about how you could most easily make the part in SolidWorks. Measure the dimensions you need to make it that way. Make the features and use the dimensions you measured.
Where you have two or more similar parts, you can make one to a particular set of dimensions, then make a copy and change the copy to use another set of dimensions. (You can also just make different configurations of one part, but that may just cause confusion.)
If you had orthogonal views of the parts it might make sense to insert pictures into your sketches and use the 2D to 3D tools to help make your models, but the trimetric views aren't very useful.