This is where I see so many guys going, and its really a tough decision for some. When you buy a nice brushed motor and upgrade your nimh battery, you have already spent enough to buy a good brushless system that will not only perform better right now, but it will last for many years. You don't have to spend $100 to get a brushless system.
This is a popular upgrade to the brushed motor. Faster than stock, but not so fast it will melt your ESC.
http://www3.towerhobbies.com/cgi-bin...?&I=LXMUP4&P=0
For that price, you can get this, which will perform better and last nearly forever.
http://www.hobbypartz.com/hobbywing-...ProductReviews
For batteries, you already saw these:
http://www3.towerhobbies.com/cgi-bin...?&I=LXVTE5&P=0
but how about these, which will give you lots more power, lots more runtime, lots more cycles, lighter weight, and more.
http://www.hobbypartz.com/83p-1800mah-2s1p-74-20c.html
The one disadvantage to these products is that you will need to change connectors for the battery and ESC, where the Duratrax pack comes with the same connector as your ESC already.
For your suspension you can pick up the Associated option spring set. This will give you a few different levels of springs to tune with. Also you may want to pick up a few different shock oils. This will change the rate at which the suspension moves. I think it comes with 30wt oil all around, I would get a small bottle of 25wt, 30wt, and 35wt. These will speed up or slow down how fast the suspension reacts, while the springs will give you more or less firmness.