I think it's generally a good idea to pot up one stage at a time, partly to keep the plant firmly contained and supported. You could always use your big planter as an outer pot and put the plant in a smaller pot inside it for the meantime (this also makes it easier to move around if required).
We have two acers of this sort (the dark purplish ones) in pots on the patio. The inner pots are about 25 cm across. They've been there for 4 or 5 years and at first I put some pansies around the edge to fill up the pot and add some winter colour. Now the roots of the acers have filled up the pots, so I've stopped trying to squeeze pansies in. The plants are now about 1 to 1.5 metres in height and width. They look splendid despite being a bit potbound and in a hot, sunny spot.
I use rainwater when it's available but I'm sure I've used tap water at times. Ours is soft water so it may not make much difference. I don't think we used ericaceous compost, just ordinary multi-purpose. I do give them some sort of feed from time to time during the summer.
What is particularly pleasing to me is that the trees were a free (p&p only) offer from the Radio Times.