Axle mount rear mechs...

[MartinC]

RT, is right. The hanger isn't part of the mech and can be transferred from mech to mech.

 

[RecordAceFromNew]

Ideally I'd like an 8-speed one...

Actually if you stick to the same brand, rear mechs are "generally" interchangeable for 5/6/7/8/9/10 speed, even for indexed shifting, it is because "speed" is mainly driven by the shifter. See

http://www.sheldonbrown.com/speeds.html#hubs

The important thing is to ensure they have the same "shift ratio" if you want to mix brands, which is sometimes possible and is well explained in:

http://www.ctc.org.uk/desktopdefault.aspx?tabid=3946

If your existing one is the same as the one you have shown, it looks very much like a clone of a Shimano model (and Shimano rear mechs are by far the most common in the world). If so practically all modern Shimano mechs should work perfectly.

However, I think very few high spec mechs are available in such "axle fit", "with bracket" or even sometimes called "hanger fit" (yes the terminology is confusing!). I have looked into it a bit before and if you want the best one, the best I could find that was offered in this "fit" was the Shimano TX71B, which e.g. is currently listed on ebay id number 330388895495 - it has a wheel type cable guide that makes shifting lighter, and this model can also handle an extensive sprocket range.

Hope this helps.