I will use this thread I have created to recommend some hop-ups that the M-chassis can benifit the most.
1) Get a ESC don't bother with the MSC the M-chassis comes with in the long run you will want a ESC anyway. Plus it is alot easier to set up.
2) Tires. The stock tires are provide good grip but wear somewhat fast. Especially if you are using a more powerfull motor. HPI makes some good tires that will fit teh M-chassis wheels. They are called X-pattern mini's they come in both a normal and a belted radial.
3) Bearings. You can get teh Tamiya bearings but duratrax makes some that are jsut as good if not better.
4) If you have a more powerful motor you are going to need a Ball Bearing Differential. More so if you haev teh RWD M-04L. it will allow faster and in some cases tighter cornering. A Ball diff can also give you better traction under accerative load.
5) This is last on the list as it is not as of an effective hop-up if you car is mostlye stock. It is the sway bar kit. This is not a "set and forget" hop-up. you should really only use this kit if you are going to adjust it to your track situation. infact if you can't set it up in some cases it can hinder your cars cornering performance. If you do set it up correctly it can give you a good advadtage to those of which they have non.
As with most R/C cars there is of course alloy parts. I feel with the M-chassis these are not that important. Most of teh M-chassis's are very durable. but if you do break your cars suspension alot, i.e. you could be a better driver, then alloy parts would benifit you. In any case alloy parts are more for aesthetics.