Only the front belt tensioner is enough.
If you want to race, you definitely want to get the 2 speed and the side brace. Also, foam tires and anti-roll bars would be good to have. I don't know how many of those items transfer to the R40 (probably none, except the engine).
Keep in mind, though, that the rs4-2 is a rather old chassis and even when fully hopped up will have a very hard time keeping up with the more 'pro' cars. Back in the day when the rs4-2 was current HPI also offered an rs4-2 'race' edition. You may want to see which option parts are on there.
Look
http://www.hpiracing.com/graphics/catalog2k2/18-19.jpg and
http://www.hpiracing.com/kits/nitroracer2/nr2-m.htm .