looks like i will need at least 2 for back of kombi not front seats if that makes a difference, or should i consider putting in sliding windows?

You can put plain one piece glass in with new rubbers and you will definitley have no more water leaks if it is sealed up properly.

If you need to have some sort of opening window, keep your eyes peeled for unrusted vent windows as the sliding windows are probably harder to get.

You do have the option of either, depending on what you can get.