Hey Rich,
firstly TOO CHEAP!
Now that's out of the way haha
you say width of shower tray is mosaic, is that one wall or two? (Sorry if I misunderstood)
have u used tanking kits before? If not I'd agree with PlanTec or go for tanking sheet on a roll like Schluter. (Schluter from CTD) much easier to use if you're a first timer and u can tile immediately. They have slightly different adhesive requirements tho.
Have a chat with Stef or Charlie 1 or any of the other guys from Scotland and try find going rates local to you. Being Edinburgh I'd imagine it'll favour u with slightly higher rates than elsewhere.
Does your friend know your skill level? If you're gonna attempt perfection does he realise it's going to take u a little longer? If you're competing against others for the job, yeah do mates rates to get work and the experience, BUT don't cut your throat. Knock 10% off not 50% otherwise u'll be under pressure before u even arrive. That's not advised especially as it sounds like your first major project. Be prepared for it to take longer than u estimate, cos it will! If it was me I'd be tackling the mosaic first, at any point do the mosaic butt directly against the wall tiles, or are they separated by the shower screen? If they do, then u'll have to consider any thickness differences, but don't be scared of them, they will bite if you're not confident. Treat shower as it's own job. Once the tough bit is outta the way it'll be all downhill!
Whilst i agree u should turn down some work because of individual skill level, I don't think so in this case. U've been cautious for a good while and not over stretching urself so maybe it is time to step up! But if you're unsure about something, STOP, get on here and ask don't plough ahead. U'll get more respect for checking than u will messing up!
Good luck Rich.