Everything you've found for om615/616/617 (w115 and w123's) on this matter applies for om621 too. They use the same collars (hence the tool,thread M24x2mm ) and the thread on the PCs is the same (M22x1mm).

You can build your own tool for the pc threaded collars out of a proper sized socket ... and for the pc removal you can use a bike crank tool + a slide hammer (google the threads on this forum for further info).

The tools themselves are not too expensive . One can find a taiwanese replica of the collar tool for $30-35...the PC threaded ones(for use with a slide hammer ) are more expensive(roughly twice the price).
Baum Tools ,Sir Tools , ZDMack , Klann ;Shimano or Park crank puller tool...
I'd make sure I had the PC rings of the right height too...and the glow plugs-removed before the intervention.