OB may choke (though I have had 3m+ polys in one cell before) but hopefully TESV will be more robust. NIF file format limits each NiNode to 64K tri's but there doesn't appear to be a limit on the 'number' of NiNodes. That textured model I posted is ~250K. Any idea what the poly count of your models are? How big are the *kmz files? Do you have an interest in getting these inagme?
Here are two shots of the Hammerfell Business.
http://www.mediafire.com/imageview.php?quickkey=x2zx2jzxzk3
http://www.mediafire.com/imageview.php?quickkey=mqti2juvmo2
http://www.mediafire.com/imageview.php?quickkey=lnywftouzzo has 4500 polygons. It's fairly ready to go if you're in the mood for another one shot building.
These are about as far as I got in detail on the city. One of the larger models has 35,000 polygons.
http://www.mediafire.com/imageview.php?quickkey=ydzdtvh2mgn
http://www.mediafire.com/imageview.php?quickkey=ncy3mw2dkyz
http://www.mediafire.com/imageview.php?quickkey=mnnfimknwzx
http://www.mediafire.com/imageview.php?quickkey=arzmyt2ydmi
http://www.mediafire.com/imageview.php?quickkey=titjxwmdizt
http://www.mediafire.com/imageview.php?quickkey=mtydtkm0iof
The http://www.mediafire.com/?cjrmdjmlnim came out to be 12.52 mb.
I think the overall city is too much to take on in game. It would replace everything about Leyawiin and a large area around the city, and then require a ton of the river Niben to be changed to suit the mega-city. The amount of time it would take to not only model the city, but replace all the npc movement paths and fix the quests associated there, it's too much to handle.