We have had lots of problems with the 307 2.0 HDI. We only have problems with RHR engine code which is the 136 bhp.
They suffer alot from turbo's not working but also the turbo actuator pressure sensor which is a pain to fit. The symptoms we mostly have is that the vehicle whilst being driven will hestitate on partial throttle in any gear around 1700 rpm. When you have the bonnet open with the car stationary, if you hold the revs around 1700 rpm so the turbo starts to boost. The top intercooler pipe will start to bulge as the turbo starts to hold a little boost.
They also suffer from the small vacuum pipes splitting which connect all the turbo pressure sensors and EGR pressure sensor. Also make sure the black plastic housing which bolts to the inlet manifold is clean inside around the two butterfly's if if has the hot intake and cold intake. We have had to change them on Volvo's which use the same engine.
We have not had much problems which Air Mass meters but you never know. Keep in mind if the vehicle is running a diesel particulate filter. Peugeot have been using them for a while. They have been known to block.