The problem about the files is even more of a problem as the ketchup exported only exports .tga files, so besides being a lot of images, they are also exported in a very large file.Lowering the distance may help with the lack of LOD but then again there's that trade-off of losing the distant scenery. I generally go no higher than 10km unless there are distant mountains to see. I find that it's a waste of resources if I'm driving mostly through close-up urban scenery and tree tunnels as it is where I live.
That's a good plan with the Sketch-up models. We were all caught short by them when they became available through the converter. I "created" and uploaded a ton of buildings and so did a lot of others. As I said, using a few here and there is fine due to the too much of a good thing effect.
Large .tga files are a more recent issue. There's a possibility that the content-creator used a higher than necessary resolution for the textures. I've had buildings load really slow when I've previewed and sought out alternatives.
The only content-creator I know of that fixed up Sketch-up models from the Creator Warehouse is RRSignal. He took the buildings found there that he uploaded and worked with them. There may be others that I'm not aware of.
If you are looking for large city blocks, I recommend Dave Drake's (dmdrake) Cityscape series. There are some on the DLS.
I am not sure how the models of Dave Snow are created, but I also like them a lot.
Do you know if there is any chance of merging the textures into only one? I am a beginner in blender or in any 3d editing program, so do I have any chance to make them better as RRsignal did in an easy way? I downloaded an arena recently and I looked to see how many images had and were over 300. The model is great and would make a great effect but 300 images!! too much