My 96 855T is nearing 200K, and I just did timing belt, serp, all rollers and tensioners and the water pump. fcpeuro has a video that helps with the project. I would second the motion on sticking with 'blue box' parts, exc the belts which you can get from Continental. tascaparts gives great discount on OEM or mix and match from fcpeuro. also, I splurged and bought a new OEM metal belt pulley cover that wraps around the bottom of the crankshaft lower pulley. Under $10 and mine was corroded. Be sure that you pretension the belt properly before releasing the hydraulic tensioner for the TB. My belt had a bit of slack, and ended up being a few teeth off, causing me a weekend of headaches till I got it right [each time you take the hydr tensioner off, you have to compress it with a vice, etc etc] When you rotate the engine to check the TB, do it with the sparkplugs pulled to make it a lot easier.
Also- some might disagree, but I felt that the $25 for a ipd cam lock tool was money well spend. Mark all the timing marks with some white out too.
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The Family Fleet '90 745 16v; '90 745 GLE; 96 855T; 99 XC, collectively approaching 900,000mi and going strong.