Evan mounts

Do you need to buy previous mounts to get the 120 one? (or whatever level the last mount is)

eg: I don't buy lvl 80 saddle and I get to 120, can I do the 120 saddle quest?

You have to buy them in order.

Yea, and sadly if you hit 80 and don't buy the mount then you can't ride on your lv50 one.

which is pretty logical seeing itd be strange riding a lvl 50 mount at lvl 80 or 120. however it would be more logical if they just upgraded the speed when you buy a new saddle, and upgrade the image/looks of your mount automatically upon hitting 80 or 120

When you hit level 80 your mount level automatically increases - your saddle doesn't. I assume this is the same for the level 120 mount, and I don't see why the quest would force you to buy a level 80 saddle when there is no need for it. The quest is just for the saddle - not the mount.