Not all the standard R65 bikes came with the fork lowers for the right side disc brake, but many of the north american bikes did - like yours! This is a bonus, as it can be tricky to find a right side fork slider with the mounting lugs if you don't have one. Basically, you do need the rotor, rotor carrier that bolts to wheel hub, calipers, some top-hat shaped spacers inserts that slide into those lug holes, and brake line to go to that splitter.
Depending on the size of your master cylinder (size in mm is stamped on body of the metal base, sometimes on the bottom) you may or may not need to change that out. The 79 bikes had a round plastic reservoir m/c that was either 14mm or 16mm piston diameter (depending on whether setup for single or dual front discs). The later models had rectangular m/c assembly that is still currently available (new parts from dealer, and rebuild kits) while the old round style are only available on the used market and they are not technically rebuildable, though some have managed in some cases.. The later rectangular units came in 13mm and 15mm, again, for single or dual disc setup, but some people prefer running the smaller m/c with the dual disc setup as it gives them more lever travel and is easier to modulate. With 15mm m/c on my LS with dual discs I cannot squeeze the lever anywhere near to the handlebar with normal 1 handed grip, and the front wheel is easily locked up with only 2-3 fingers. Some prefer less rapid progression.
Recommend you pitch the old rubber brake line and get some new stainless steel braided lines - you can order premade sets from bobs BMW in Maryland, they have someone build them to order. Or you can get the OEM BMW lines, but the SS lines definitely seem to offer a bit more "bite".
