So far all Dutch 210 drivers I spoke to add weight to the rear of the car and try to reduce the amount of steer of the car, whereas I heard from them apparently many UK 210 setups add weight to the front... Since you're used to a rear motor car, perhaps it helps to bridge the gap a bit (for now) and add some weight to the rear and/or run a shorty LiPo as far back as possible?
What should (theoretically) help to reduce the twitchyness is a plastic tub chassis or a carbon chassis plate - people also seem to be quite fond with the longer wheelbase (not just DEX210). As this is quite an investment on a car you currently have your doubts about I would try the weight balance options first.
To solve your oversteer... there is a lot to try, especially on the DEX210. Could you tell us when it oversteers: corner entry, mid-corner or out of corners, and if this is just a low speed problem or also at higher speeds?
