I own a micro RS4 in basically stock form.
The best thing you can do with this car is lighten and lower the electronics. In stock form there is very little room to mount a full-sized receiver and speed controller. If you are looking to get one used, make sure that it has a quality small speed controller. LRP makes small speed controllers that are good quality. Novak makes the SPY which has the stock connectors for a micro battery and motor.
Some of the mods you will see are chassis and motor mount. Honestly, the stock motor and 4 cell battery tray = slow car. However, you can purchase the HPI mod motor and a 5 or 6 cell micro battery pack and really improve performance. These 2 upgrades alone can turn the car into something truly fun to drive, but aren't too expensive for a racer on a budget.
A lot of people put a larger motor into their micro, which usually requires a motor plate upgrade. Speed 300 motors are used a lot, but need a bigger motor plate to mount them to. If you are buying used this might be a bonus to look for.
Another major upgrade is a pan chassis for the micro. Since the stock double-deck chassis doesn't leave much room for electronics, a better chassis is worth looking into. I've seen various aftermarket chassis for the micros, but they usually share one thing in common - more room to mount your stuff. I won't suggest a specific chassis, but a few I know of are Ratzazz, Penguin, and Hardcore Racing.
Even with a stock chassis and a good motor and batteries, the Micro RS4 can be a good car. While the tuning options of the car in stock form are limited, there are some things that can improve performance. The biggest key for this car is to find a smooth surface to run it on. A clean parking lot or street is a good place to run the car. Since it is belt driven, you want to avoid dusty surfaces with small rocks.
Since I'm not a real micro expert, my best advice is to try some of the dedicated micro forums. A good one is
http://www.yourmicro.com.
I hope this info helps.
