It does look like the cladding goes straight onto the wall and old tiles but if that is fine then I'll take it. Just noticed that the shower drain is going to be at the opposite end of the actual shower, should it not be at the same end?
The side with the drain will most likely have the fixed part of the shower screen whilst the shower head side would have the shower door. This way you're less likely to stand on the drain cover and split it, also the shower head water passes the door opening as it projects into the fixed closed in end of the enclosure. You also wouldn't have the door on the drain side in your case as your toilet looks as though it's going on that side.