Sorry dude,
I was up visiting MrDave and the caddy to get my ALH this weekend. I've looked on other forums, but haven't been able to find much on the AHU swap.
I'd have to say if you already have the engine, pull the old one and mount it. If it fits on the A2 mounts and you don't have clearance issues with anything like the turbo, you're in decent shape (the major issue with the ALH setup). Next, if your axles mount up (100mm on the A2, not sure on the AHU-A3).
Finally, the electrical demons. If you've got a full harness, you will save yourself some (not all) psychiatry bills.
What do you have for the swap, if anyting?