Replacement is fairly easy, but the tensioner is the sticky bit.
You can test the tensioner when you remove the belt, but if you have 100,000 miles plus on the car without a new one, expect to get it. I'd buy one anyway, they don't go bad sitting and you will then have it if you need it.
Changing the belt if the tensioner is good isn't bad:
Remove fan and clutch, along with radiator shroud. Loosen the large (19 mm I think) bolt on the tensioner and unscrew the adjusting nut beside the water pump. While doing so, watch the pointer on the little "ramp scale" on the tensioner. If it moves back clockwise to the line before the ramp, all is cool and you can re-use it.
If, however, it goes only part way back or doesn't move, the tensioner is kaput and you will have to replace it.
It's held on by the big bolt in the center, and a bolt on the right side of the bracket, two on the other. You must remove the adjusting nut. Make sure you get the adjuster on correctly, or you will snap it. You also need to remove the tensioner shock.
Installation is the reverse. Loop belt around the pulleys, and remember that the shock will push the tensioner arm all the way "tight" and will have to be compressed manually to get the belt on. Use the adjuster to pull the pointer on the tensioner up the top of the "ramp", then tighten the big bolt.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!