If you plasterboard wall, then you can fix hardboard to the plasterboard, then tile on to hardboard, this is so if you ever decide to remove tiles, it is easier to remove from hardboard and does not damage the plasterboard underneath.
Hope that answered your question, good luck
I'd always put a plasterboard up, it will give wall a bit more strength as hardboard very flexible and only 3-5mm thick, and I guess you may want to fit enclosure up and may be shower too.
I would prime wall with PVA but don't let it set just go tacky before appling adhesive.
Best of Luck
I have heard of this, there is some sort of chemical reaction that causes the pva to fail, fortunately I have never experienced this and I have tiled, dot and dabbed, plastered many a wall in my time using PVA and touch wood had no known problems!
You can buy primers that work well with tile adhesive if you have any doubts! I don't think the costs would be a great deal different?