I think you should spend a lot of time reading first. And define what specific need you have for this engine. Because while you can increase the output of these engines quite a bit, you bump up against a problem with poor driveability, reliability, durability, as well as all the other items the engine needs to get the power to the ground suffering.
The AHU is an engine that was dated when it came out, based on a design from 1974. The ALH is a fresh start, and is a superior arrangement overall. They do not suffer from the weak crank snout since it is beefier, the flexing of the lower block is less concern since the cast oil sump is also used to make the bellhouse a true full circle. They run a single poly belt for the accessories, closer to the front of the engine. The timing belt is thicker and more durable. The injection pump is better, and has easier choices for more power options. The lubrication system is totally different and improved, no leftover bits to drive a mechanical gasoline fuel pump and ignition distributor for carburetted gas versions like the AHU still carries with it. The ALH's head also flows a wee bit better, and has better valves, valve guides, and even the ALH's pistons are improved. Yes, you *can* upgrade a lot of the items on the AHU to bring it to that level, but the ALH is already there.
However, if you already have an AHU, I understand why you'd want to use it. But, with your lofty (unrealistic, most likely) goals, the costs involved should be of no consequence, and doing the work involved to put a newer engine in your car should not concern you.
The A3's chassis isn't well suited for that kind of power either, and will need a lot of tweaking and improvements just to be able to handle it, and it'll still end up being a white knuckle ride that will try every chance it gets to place you in a ditch.