An extra spur gear is a good thing, but not necessary if you are experienced at setting your mesh properly. Bulkheads are pricey, and also not necessary if you are an experienced driver and steer clear of head on collisions with parked objects while WOT.
The obvious things you might need to get things going in the right direction:
1. Quality air filter; I recommend the MotorSaver series
2. In line fuel filter; DuBro and Dynamite are a couple of companies that make them.
3. Fuel of course and some batteries; I'm guessing that you have those already.
Hop Ups to look into:
1. ProLine Suspension Kit; Yes, Matts got that one right. You could get the same width addition by going with an nice set of RPM or ProLine rims with wide offset. I run a set of Monster Claws by RPM and a set of Velocity Six by Proline. I am happy with both, and have not purchased the Pro Line suspension kit. If you get both the suspension kit and the rims with wide offset, your T will be almost un-flippable.
2. In lieu of the Proline kit, RPM A-arms. They are virtually indestructible and if you manage to break one or all, then RPM replaces them for the price of shipping and handling.
3. Titanium turnbuckles and hinge pins. These things tend to get snapped or bent often. The Ti sets will give you a little more strength and less bend.
4. RPM anything...see number 2 for the logic.
5. Upgrade the engine when you can afford it. Fantom is my engine of choice, but there are a number of quality engines out there (OS, Picco, etc.)
6. Upgrade the steering servo; I recommend the digital hi torque servos that HiTec produces, specifically the
HS-5645MG.
7. Aluminum chassis braces and center skid are a good investment for the bashers out there. Stick with the plastic skids or upgrade straight to Titanium skids for the front and rear. The aluminum ones will take up most of your time in rebending them after nearly every collision.
8. If you switch to a steel spur gear I recommend getting an upgraded slipper clutch system. I use RRP's slipper disk set up and it is awesome.
9. You can always go after a nicer set of tires (lighter weight than stock..recommend the Maxx mashers).
10. Better shocks. Go for the Big Bores or Associated Engineering. If you can afford it go for Progressive's set. They are way pricey, but are nice for all driver's styles. Regardless of the shock upgrade, also upgrade the springs. I recommend Trinity Blue or Black (depending on the weight of the Maxx in question...blue is a heavy set, black is an extra heavy set...you could run a combination for the in betweeners). You can also look into RC Raven's Dual Rate springs...ask Christian for his opinion on them.
Spare Parts:
1. Extra glow plugs; I suspect you may have some on hand
2. Rod ends for the turnbuckles, especially if you go with Ti turnbuckles. They tend to give out after a while...particularly if you are a hardcore basher and fond of big air.
3. Screw set for replacing the bent/broken/lost screws. Racer's Edge makes a nice set for about $35. It is a complete replacement set for all the screws on the T-Maxx.
4. Body clips...those things tend to disappear.
5. Shock oil and a rebuild kit. This is for periodic maintenance.
6. Air filter elements...for those times when your current one gets too dusty to handle the air flow...then you can replace and go vice having to stop for the day as you wash and re-oil the dirty one.
Sorry for the long response, but wanted to be thorough. This list is not all inclusive, and I'm sure someone will have more to add. It should be a pretty good start on what to look for.
