Batton with 2x2 battens and add 50mm kingspan between with plasterboard over, will help no end. is there a flat roof or a pitched roof above? if pitched, is there access? Is it insulated?
Put a little bit of expanding foam in the gap and when it sets trim it off just below the surface and apply a skim of easy fill over top, sand back and decorate.
If you want to save space and hang heavy tiles then have it rendered throughout. is 3mm really going to be an issue to you if you use 12mm boards? It would be worth checking with the company that makes the tanking if their product has any weight limits as you'll be bonding your tiles to the...
I know how to imagine, i also know that a picture would save a lot of questions and not having to describe several scenarios.
So do the joists run along the wall that was removed or do the floorboards?
agreed with Steve, apply it about a foot out from the edge of the lip and feather it away, it can be easily sanded after with 180 grit paper and you will never know, give it a good few mist coats after a few days before top coat.
Leave alone and paint before you sell, deal with leaks if they arise, your only likely to get leaks through if there are cracks in the render and even then you'll have cavity walls, unless they've been insulated with fluff but thats another matter!