It's been a while since I used a Dulux One Coat gloss paint, it was awful!
It may have improved since then but I wouldn't specify it on a job. If you use the water based type be prepared for a low level of gloss and ensure that there is thoruogh preparation especially if going over oil based paint.
I think it odd that a 'Pro' wants to use a retail (ie non trade) paint, I'm sure One Coat is only sold in retail tins.
Whatever you use, good preparation is the key to a good long lasting finish. It's the hard dirty bit, the bit where short cuts are taken!