How do I replace the Bendix on my starter for BMW 318i 2000, illustrations perhaps?

Replace the s/motor with one that has the same number on the outer case . Or join bmw forum for more info on your car .
No owner should vwork on his car without first purchasing a manual .The BMW forum is a must for you to find answers .

is this an engine swap? a fraken Toyota?

roblem with 2000 Toyota RAV4 Cant find the timing mark after timing belt was cut, 1991 BMW 318i E36

pull front nose of starter by unbolting it and then pull cir clip off then slide bendix off and replace with new one

The "Bendix" is the starter drive in the actual starter. When you crank the starter the bendix is driven forward into the flywheel. It sounds like the bendix spring is worn or week and it's not meshing with the flywheel. Instead it's grinding against it. If you're ambitious you can pull the starter, dismantle it and replace the bendix or option 2 replace entire starter w/new or rebuilt. Both will have new drive and save you a bunch of time.

in the boot on r/hand side.

Either the fan switch on the engine or the fan relay is at fault. Replace both for they are inexpensive to replace

As long as it's the same part #. Only the driveshafts differ between the two models.

One of 2 things. The part on the end of the starter called a bendix is bad (most likely and less expensive) or the ring gear that the bendix engages is bad. (less likely MORE expensive). If you are comfortable with the work try locating a starter locally and replace it yourself.

generally all starters have two bolts that hold them in.  Plus the associated wiring.

I would have the battery tested. When they get cold they get weak. Next I would look at the wires on the starter to see if their loose.