Simple IAP works for XBOX as XBOX uses the windows app store for its transactions.
For achievements you could still make them using Playmaker and an array to hold them, with pop up windows, etc.. When you are ready to jump into the Xbox side you can then create a FSM that links the two together. IE. when you earn the achievement in local game side you can fire an event to send it to the xbox side.
Just an option for you. I know it doesn't solve your question, but this would allow to you code in all your achievements, build your game and then look again at assets when you are close to completion. Assets change all the time so something better may come along.