path follow with boxes as path.

I can not figure this out by myself. banging my head for 2 days now.
Basically it is a path follow with boxes as path. I hope I can explain correct.
I want object called mover to go over this path.
If box_001 is selected the object mover is on the same place as box_001
On press but_up pressed
box_002 gets selected and the mover.position = box_001 then mover.position = box_002 and adding key to time line + 5
and so on..

I am not sure how to achieve this cycle in objects.(From box_001 to box_120 )
I tried braking up the name extension and replace the last 3 digits
so that I get: box_
then adding value back to the string. When button pressed it should add a new value (instead 001 it would be 002)

Problem is that when I do this it will get just a 1 instead of 001 (for example)

This is what I am after for this part :
make 2 buttons
1 for going up. (but_up)
2 for going down. (but_down)

--If i select box_030 and press but_up it selects box_031
--If i select box_030 and press but_down it selects box_029
--also needs to cycle from box_100 to box001 and reverse like box_001 to box_120

would be very nice if i could get a spinner in the tool so that the back of the name extension is controlled by the spinner value (As selection method)
off course it then needs to have a selected object first so it knows the value to begin with.
that can be a pick-button.
then it would be possible to jump selection from box_001 to _box_030 at once. with all the data calculations that would be in between. like the time line adding.

Hope I explained understandable,

please can some one help me with this,

Kind regards

Ronny