Belts & Pulley Replacements/Noise?

[nelsontoml]

HI all, I have a 2000 X5, 4.4L, with about 130K Mi on it. Started getting a squeaking/whining noise in engine area, sounding like belts. Sprayed Belt dressing on the belts and it got quiet for about 30 minutes of driving, then started back. I have good mechanic that said its likely time to replace the belts, tensioners, and idler pulley. But he said to go online and order everything and he'll come over and do the repairs. I need advise on who to order from, and exactly what belts, and pulleys, etc i need? I'm thinking that as long as im doing one belt, i might as well replace all needed belts and pulleys at the same time, correct? and is there any other items i should consider now while doing these things?
Thanks for your reply!