Have you considered a Reed? It has buttery rich flesh that oxidizes very slowly. You can leave cut fruit in the fridge for a day without it turning brown. It's also a columnar, relatively smaller tree. There's lots of other rare varieties as well.
Check out this website
http://www.epicenteravocados.com
Simon
I've been planting, torturing and killing avocado trees in my back yard for over 15 years. I'm sure I already had a Reed that died a miserable over watered death in one of my half whiskey barrel patio pots.
For some unknown reason, my luck has changed and I've managed to find places in the yard that have good drainage and counteract my tendency to drown the trees.