Is there any easy way of directly jumping to the definition of a Variable like it is implemented in some good IDEs? Currently I have to search manually (ctrl+f) through the whole workbook to find it somewhere.
If you use package files, ie, files with extension .m which contain mathematica code in plain text; create one from File->New->Package:
there is a button which lets you jump to the definition of symbols:
Clicking it shows a list of functions:
(although it gets confused sometimes, as you can see in this screenshot); clicking on a symbol name usually takes you to the assignment (if eg you have them all in one cell, this doesn't work...).
Now, the m-editor is not ideal, but better than searching. Note that by default it does not keep the indentation when you press return; this may be changed as described here. Also, block-indenting in this editor is impossible...