You say your members will be either in tension or compression. This is not entirely correct. The members supporting your UDL's will be in bending. If you design the entire structure as a pinned frame structure you will end up with the remaining members being either in tension or compression. In that case all your nodes have to be pinned to avoid moment transfer across the nodes. If you want to avoid designing for bending (except self weight of the member), your 'base' nodes would be all your corners around the perimeter of your frame as well as locations where the supports and any point loads are.
You would then have to look at the sectional properties of your members to ensure you don't end up with slender members for the compression members. This is the one of the reasons you might need an iterative (trial and error) process.
You would start by dividing the structure into triangles. Something like the image below. I think that these members would be the bare minimum.