Parts: There are various kinds of parts as well as sources. Prices can vary as much as 100% across suppliers. You can do very well with name-brand (Sachs, Pierburg etc.) parts from name-brand suppliers (Autohaus, Bimmerparts etc) if you learn how to look. At the end of the day, the only thing that really protects you from counterfeits is the reputation of your supplier.
• BMW vs. OEM vs. non-OEM vs. Aftermarket: Yep, there’re four major categories, and no one explains them to newbies. BMW is the thing from the dealer. Costs twice as much as non-BMW. Most of the time, the only major advantage over the OEM part is the logo. Ex: BMW alternator made by Valeo: $800. OEM is the BMW part made by the part manufacturer who did it for BMW. Same part, no BMW logo. As good as BMW for half the price or less. This is by far the preferred source for DIY support on the E60. Ex: Valeo alternator for BMW’s: $<400. Non-Aftermarket: This is the part manufactured by someone other than the OEM manufacturer. Quality varies widely, from just-as-good to iffy – so it’s crucial to order your parts from a known source. Ex: Alternator from chain store: $100-200. Performance (Aftermarket): Usually performance or upgrade parts, hopefully from a name source. Ex: Koni or Bilstein shocks designed for your E60, or a performance exhaust, or monoball plastic bushings for your thrust arms.
• Finding Parts: Best: go to RealOEM.com, learn how to use it and get the BMW part number. Google it. Search name parts houses, buy the lowest OEM price. Next: Download PNPC and search by part number across 40+ locations. Then pick the best price plus shipping. OK, but tiring: Search by vehicle type on whatever house you like. Always include the last 7 digits of your VIN when you order. Parts often change over time as the E60 evolved, so you may get the wrong part. Learn which parts are BMW OEM parts, like Lemfoerder for suspension or Sachs for struts/shocks, Pierburg for electric water pumps, Valeo for alternators, Bosch for starters and so forth. See the illustration below.
• Source: Good BMW supply houses will clearly identify OEM or not. Oembimmerparts.com is a place that clearly does this. This writer won’t buy anything important on Ebay or Amazon. Sure, air filters, lightbulbs, perhaps, but nothing that matters to operations or safety. Your choice is yours, of course.
• Another option: If you’re sophisticated enough in the process, you can find parts sourced for the OEM version of the unit. For example, a Valeo alternator can be found in established alternator specialist supply houses. So can the regulator that’s on it. So, BMW alternator: $600+. Valeo Alternator: $350-400. Regulator on the Valeo alternator (the part that usually fails) from an established supply house: $30.