Thank you for your responses!
After spending the rest of last night playing around, I actually managed to cobble together a shell mesh of my prim building. Test uploaded it on the preview grid and it only costs 4 prims (and that's without doing any work on the LODs). Original prim build was 40+ prims, so I'm very pleased!
Also, I'm happy to report that if you set the scene measurements to metric, the Collada exporter in Blender 2.5 will honor them when uploaded into SL. Scale is 1 to 1!
Also, in case anyone else is looking for this bit of info, I found that you can in fact precisely move/extrude with numbers. What you do is after grabbing or extruding, you just type in a number (pos or neg) and the selected verts will move that many grid units away (meters if you've set your scene up that way). Click or press enter to confirm.
PS: Last time I played with uploading a mesh, I would get two assets: a mesh asset (yellow prism icon) and and object with the mesh applied to it. Now I only get the object. Did they do away with the mesh inventory assets in the latest dev build?