You can lower your AWD with The KW V2 kit including springs and coilovers. They are adjustable in height and in hardness. So you will be able to choose your level of lowering and comfort.
You will loose a little bit in comfort, it's inescapable since you lower your car, but depending on your setup, you can loose not that much.
You can find this kit on ebay for about 1200$.
OR what your can do, is buy just shorter springs, like Eibach make. You will almost not loose nothing in comfort. But I suppose that you will have to replace your stock shocks more often. This solution is the most appropriate for you, I think. And it's cheaper than KW kit.
Make your choice !
You don't have to change nothing in suspension for 20's, because you will put low profile tires like 245/45/20 which are same exterior diameter as the stock 18's.
I'm on 20's and I didn't feel difference in performance.
so you can go with 20's easy !