First off take some cash just in case the price is a little keener
The rollers will give you a power reading for BHP and torque which will give you an idea of how well the engine and turbo are doing. The torque value will be in NM rather than the usual lbs/ft but you can use a
conversion tool to give you both values. They should also check the fueling with a wideband lambda i.e. probe up the peashooter, although best to remind them that you want the fueling checked. They use a lambda value for fueling so you should be looking at 7.8-8.3 when on boost for good safe fueling.
Finally they'll print out a graph so post it up here for further analysis along with any other info e.g. fueling result