You may be able to pack out the stud work to help get the drywall sections reasonably flat. This can save a lot of messing about with a screed. Also, if you are fixing tiles, unless you are Italian that is, some variation in plane isn't really that noticeable and the tile adhesive can be used to pack out a millimeter or so here and there.
If you are doing a complete wall of any size then I recommend investing in a plastering float, assuming your trowel is what I call a trowel.
PS. Don't forget to fix your first line of tiles just short of two tiles high from the floor!