W140 S320L underside corrosion

I'm looking at a 1999 S320 W140 Limo.

The MOT advisory notes state that:

1) fuel pipes corroded
2) Front under-body has slight corrosion
3) Centre to Rear pipe slightly corroded
4) Rear brake hose has slight corrosion to ferules
5) Oil Leak

The car bodywork seems to be fine from the pictures. Does anyone have an idea whether the corrosion on the pipes and underside is a big job? How many labour hours should I allow for it?

Also, can the oil leak be symptomatic of the head gasket going at some stage?

Any info and background knowledge will be much appreciated.

Many thanks