On the subject of hoses, I don't know of a set interval, but my 20 year old TD still has it's original hoses. My first Mercedes was a 1980 300SD. I bought that car when it was 20 yrs old. It to had it's original hoses, which I promptly replaced. When I removed them, the insides were perfect, not a thing wrong with them. Remember, hoses rot from the inside out. I think as long as the hoses don't have soft spots and they look good inside, keep them on.
1994 MB E320 Coupe (199,500 mi)
1992 MB 190E 2.6 (173,000 mi)
1983 Porsche 911SC (148,000 mi)
2004 Volvo V70 2.5T (198,000 mi)
President of CT/Westchester Section