O god... I could go on for hours if you have no limit... although, I know there is a limit so I will do some of the more important things...

First I say get a good FM radio. The stock radio is the worst parts of traxxas... I hate their tq2 system they have(maybe a little different for older ones?). The Airtronics mx-3 is a good reliable FM radio. Also the JR xr3i is a very good radio, my friend has one and is very similar to the mx-3. There are many other good radios, these are just the ones I have personal experience with. There is some new one that uses cell phone technology, I have only heard about it, and never read about it, so I don't have that much information. I'm pretty sure it is very good, and very expensive.

Next thing I would say is a good engine. You shouldn't need anything fancy just to put around in a field or backyard, but if you are considering racing, you might want to consider a more, high-end motor. Maybe even get an engine and send it to RB Mods ( www.rbmods.net ) and have them mod it for you. You do not need this, and it will void your warranty on the engine... but man, I think It might be worth it... It is also out of most peoples price range to do this(at least mine[&:]).

Tires... I feel this is one of the biggest factors of having a good race. If you have worn out tires, or improper tires for the track, I can almost guarantee will will not win a race in A main. But if you are not racing, this is not as important. Although it is usually allot more fun to have a car you can actually control while driving it...

Now time for the shopping spree of upgrade parts for you car.. You can go a few different ways, there is the aluminum route( the road I took), carbon fiber, or as many do, RPM.
Aluminum-aluminum is definitely heavier than plastic, graphite, or CF. Although, it is much stronger than plastic, much stronger than graphite, and about equally as strong as CF(in this case). although aluminum bends in stead of snapping like the other parts would, this makes it so you can bend the piece back into shape. This also weakens the aluminum, so next time it won't take as much force to bend it and so on. but I guess this is better than a snapped part.
CF(carbon fiber)-I believe CF is 7 times stronger than steel to the weight. Which mean a pound of CF will be 7 times stronger than a pound of steel. This does not mean a CF chassis will be as strong as a steel chassis, considering CF is MUCH lighter than steel.
CF is a very good route to go for racing.
RPM- RPM is a company that makes heavy-duty plastic parts. They are VERY strong(I thought I would break them the first day I got my a-arms, haven't even phased them yet), cheap, lighter than aluminum, and have a lifetime warranty.. very good route to go for a basher, although they do not make every parts for the rustler, so you might have to buy alum. or CF also.

Rims are also things you might want to buy, I broke my stock ones into 30 pieces in the first weak I had em, then I got rpm and I can't even chipped em...

EDIT: Woops, forgot about shocks... traxxas shocks suck, you will want new... get some aluminum ones like all the other companys use. Haven't tried traxxas big bores, but I have heard very good things about them, you might want to consider those. I went for the duratrax nitro evader shocks... they have been very good to me in the time I've had them(since before my NRusty). many different options for shocks.

Crap, also forgot. Some people went ahead and bought some MIP CVDs, I have heard good and bad things about these... the problem with CVDs(constant velocity drives) is that if they come lose, they can fly off your car and you can lose your good set of of 35 dollar drive shafts. this does not happen with the stock. I have also heard that MIP made thier CVDs for the rusty too short, this may cause some problems in the long run. I personally find the stock ones fine, but if the plastic ones just aren't doin it for you, maybe try the MIP CVDs.

Body, fuel lines, turnbuckles, etc.. aren't that important, unless you are in dire need to replace them...