Need some help cleaning up/optimizing a map

I’m working on starting a small scale build/combat/RP server, its become an interesting concept and coding has gone well so far. I’ve even so much as found a map! unfortunately the map(ww_somme) has a few pitfalls, making it partially unsuitable for my use.

-materials and models from WWI source, which need to be downloaded separately
-low ceiling
-random invisible walls messily scattered throughout the map, I think
-some skybox and incomplete terrain issues near the edge of the map… not an issue but I would like the play field to be completely usable without noclip to get in/out of glitchy areas
-no good build space… I’d like to have one flat, rectangular, grid material build space in the unused corners of each side of the map.

See, as a DM map it’s awesome, with barbed wire that causes a bit of damage, trenches, hills, etc., but I need it to also be suited for building a couple tanks, or small planes.

If someone wants some practice with map fuckery, then I’d appreciate the help, I can email you everything you’ll need if you dont want to download WWI source.

[editline]4th January 2013[/editline]