I don't use blender, so the tools might be a bit different. But the general process should be the same...
1) Polycount.
The easiest thing is - delete garbage - Sketchup just loves to pack the model full of un-needed stuff - backfacing polys, extruded edges, (not to mention all the tiny details no one is ever going to spot in tha game). Fortunately most of it comes as separate objects/meshes - so it's rather easy to get rid of.
What i usually do, is - select the parts you want to keep first - one by one - and hide them (or freeze or rename, or move to another layer). Then just select and delete everything that's left. The screenshot below shows "everethinh that's left" - after i hidden the actual hull and some other parts i decided to keep - as you see its obvious garbage - and its worht 2200 out of the 3500 polys
On this one i also replaced some stuff, that was easy to access/reattach - the gun, the tail rotor and the cockpit interior - with extreme low res meshes - concidering the game scale a 3-barrel gun is not worth the extra 300 polys.
On this model this, fortunately was enought.
But if you really need to drop the polyount even more you could go ahead and remove some edges/verts from the main meshes - reducing the smootheness, of course, but concidering the games scale again....
This last step is somewhat tedious, and often not worth the effort.
In the end i also wielded the separeate meshes together (unless i needed to apply different materials to them)
2) Do you mean how i exported that jpeg-picture showing the mapping ? Or how i did the mapping ?
For export - there is a plugin for max, that will allow to export the uvw map in a picture file. I assume, something similar does exist for Blender.
If everything else fails, you could try Render to texture using a Facetted or Wireframe material.
The unwrap itself - It is a bit hard to give general guidlines, as every model is a bit different. When applying the UVW map, its often easier to start with some kind of default mapping, and then work from there to "finetune" the texture space. I applied a planar map (from sideview) to the hull, then cut it in two to fit it in the square texture. Then stuffed the rest (Cockpit, Wings, Gun, Rotor cap) in what space was left.