#2
depends, if you have a 550 already, leave it there and use that!
#3
lol no, thats not the case.

I have an rg370dx, neck broke, looking for a replacement.

would it work?:P
theres one going on ebay
#5
Some 550s should have the AANJ if I'm not mistaken, but I do not think they are very common. You know the 550 has the same neck as the 560 and 570 right (save inlays IIRC)