I agree about the weight but a 15month old should be about 9kgs. Wasting your money going rear facing. I don't think you can get one anyway for that age. Plus, the kid is starting to get very curious now and demanding and they will appreciate the forward facing seat and so will you when you or yours will have to constantly turn around to give them attention on the move.
The Isofix thing is something you need to check alright. I think it has become standard in all new cars over the last 3-4yrs