You'll get a bit more time if you avoid the Empire's base. If you head close to it early on, it will begin sending out units at you. Also, I think the earlier you start building on the tree, the earlier the empire attacks. You may want to delay building on it for a little bit, but don't delay too long else when they do attack they'll destroy it in a couple of shots and you'll have to start over.
I usually put up a wall a little east of the Tree and build a number of turrets in there, then wall out the rest of my base so they can't get in through the back. I usually have a lot of laser troopers in near the tree, usually sitting turrets. Doesn't hurt to have mounties and perhaps grenaders to help take out artillery and mechs. If you've enough power, you might try hitting the Trandosians in the north as well, since you'll get mechs if you destroy them with can be help for fending off the baddies.
Assuming you can set up your defenses well enough to block out most people, you're still not out of trouble. The Empire has a nasty habit of shuttle dropping Dark Troopers almost right on your tree. So be wary.