The ES body kit is already pretty decent looking, and is functional. You can add a larger and higher wing, but it won't do anything but make the car look different. I have seen offset rims put on daytonas and the fenders flared out, but I've never seen a kit version that looked good.
On the other hand, I have seen one custom job that actually used flares similar to the conquest TSI fenders, and offset rims morphed into an ES body kit. Looked really cool, but I bet it was expensive.
As for the motor, if it's in good shape (and I mean really good, no smoke, no cold rattling) you can get more air (intake) to it, and buy a programmable ECM, and have some decent performance, without actually tearing down the motor.
Stay away from the cold air "resistor" kits, you'd just be wasting money, and you won't gain any "20" hp, either with one. Besides, it's just a 100K resistor across the air temp sensor.