Tip #1
If you have more defenses than you need, you are doing it wrong. Figure out what works best, and pare it down to the point that you need just that and nothing else. Put every spare penny into Banana Farms and get those things going fast. The payback is approximately 10 levels, and after that everything is pure profit. If you can plunk one down on level 16, go ahead and do it.
I don't know if you could do the last map on expert without it.
In any case, you can play around in the sandbox to see how various configurations work before you start the level. Once you know how to minimize your setup, go ahead and do it.
