Yeah Nevyn, the difference between the design that I proposed and your one is yours is a 'constant loop' system, more like a vertical escalator than anything else, whereas mine behaves identically to a modern elevator; one elevator carriage, and the system is stationary when not in use.
Poltifar, I assume the diagram is clear enough (the "I" is the halfblock-gate-gate-halfblock bit), so I'll clarify the redstone.
At each floor you would have a row of buttons, one for each level. These would each lead to a linked set of RS-NOR latches (basically circuits that can be used to save states) that are designed so that turning any 1 on deactivates all the rest. Each button corresponds to a given level, when that button is pressed, the detector torch for that level is turned on. I should note that this is really only for many loop systems; you would struggle to get more than three levels with a single piston loop, and for 3 levels, you wouldn't even need this fancy linked latch system, just a single RS NOR latch for the middle level. The top and bottom floor would always have their detector torches on for a 3 block level, simply to keep the lift from overrunning and crushing you into a 1x1m cube.
You then hit a send button to move the belt until the marker block (let's say a glass block) passes the detector torch that is currently switched on, at which point the system stops again. The 'send' could be combined with the level select button, but it'd need a little extra work to ensure that it knows to send the loop in the right direction. Because the marker is in the same belt as the actual elevator carriage, the detector torch corresponding to the top floor is at the bottom, while the one for the bottom floor is at the top.
I hope that helps. Rereading it, that doesn't seem likely, but oh well...