I bought my daughter a 06 CLK 350 and she has 21K miles and it appears that she will soon need new front tires. I realized that both her car and my SL have different tires on front and back. Hence, no rotation equals less tread life. So, I guess I have no choice and just buy tires on a faster pace?

Yes, but rears usually wear out faster than fronts unless the alignment is off. But in that case the wear would be uneven. Maybe the original owner already changed the rears and not the fronts.

Yes, I just order some myself before I get back from overseas in the next two weeks. I order a set for the F&R F:NITTO Invo 245 35ZR20 95W, R:NITTO Invo 285 30ZR20 99W so they should be there my the time I get home.