Make sure clutch is ok, make sure turbo is nice and punchy and that car deosnt smoke excessivly. As for bhp, the 115bhp has smaller turbo that 130 and 150 but is cheaper on insurance by a fair bit and when remapped is as quick as a stock 150 model. 130 is best all rounder imo, 150 has larger turbo and front mount intercooler than the 115 and 130, but can suffer cam shaft problems which are expensive to rectify.