Two things to be aware of about the Magnefine filters: it should be installed on the return line, and be aware that they sell 3/8" and 5/16" filters. The 3/8" was too big for me, and the 5/16" was perfect, though I can't offer any help on which one will fit yours. If you tow, a transmission cooler would probably also be an excellent idea. Check out the "Should I install an auxiliary transmission cooler" question in the
General Info page.
Personally I don't bother with the rock catchers in the transmission. The openings are far too large to catch anything useful. If that thing is plugged up, your transmission has already grenaded. The only positive thing I can see about replacing it is you get chance to see if there
are any chunks of stuff in it.
If you want to replace all of the fluid in the transmission, your best bet is a transmission cooler line fluid exchange (also on the general info page) which lets you collect the old oil as it comes out, and have it fill itself up with fresh oil. If you're regular and diligent about fluid changes, a drain and refill should be plenty though.
Sorry I can't be of more specific help. We don't have too many R51s on this board, which is probably a huge testament to their durability!