I used to be the acting lead of the BF40K team a long time ago where we were working on Battlefield 2 and 2142 mods.
Sadly we never finished the mod. We never released a mod since our BF1942 game.
After some negotiation with ex devs and UT40K I handed the assets over so that the work would not sit on a hard drive doing nothing.
I do not mind helping you out in an advisory role.
I may be able to get you access to all the assets that were developed for the game for :
Imperial Guard Cadian Shock Troops, Static Emplacements & Vehicles
Tau infantry, Static Emplacements & Vehicles
Space Marine Blood Angels infantry, Static Emplacements & Vehicles
Environmental Assets
Not all the artwork was complete however and would require further finishing.
We also have over 180 professionally made sound effects and 2 professional soundtracks.
It would give your team a big head start there would be strings attached before I could hand them over. I also need agreement with ex devs.
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If you would like to have a look some of the mod assets can be found in the links below.
Game download UT40K built on UDK
http://www.moddb.com/mods/ut40k-the-chosen/downloads
BF40K-1942 Requires Battlefield 1942 game
http://www.moddb.com/mods/battlefield-40k/downloads
Battlefield 2
http://www.moddb.com/mods/battlefield40k
Battlefield 2142
http://www.moddb.com/mods/battlefield40k-bf2142
Videos
http://www.youtube.com/user/BF40kadmin
Exterminatus the Source mod is worth a look too
http://www.moddb.com/mods/exterminatus
I don't know how you would handle walker vehicles in the engine though. I doubt it is purposefully designed for them.