Here are the areas I specifically had inspected when I was having the pre-buy done on our Mooney - based almost entirely on suggestions I've found on this forum and in the MAPA magazine:
Pull wing inspection panels and inspect main spar along entire length of wing
Thoroughly inspect in wheel wells. Towards the aft of the airplane in the well you can see the stub spar. The front side of the well is the main spar.
Inspect main spar right under rear seat.
Pull rear seat inspection covers. Once again you can see the rear side of the main spar by looking forward from these inspection covers. You can see the stub spar by looking rewards. This area rarely sees inspection.
Open up access panel behind passenger side of the wing (where flux detectors, ELT, transponders, autopilots, etc live) - check the structural members found in the rear section of the airplane (referred to as the "tail cone") for corrosion.
Pull inspection plates below the elevators and check the structural elements of the rudder spar (is that what it's called?). Pay attention to areas where water can accumulate for long periods of time.
Pull all interior trim to check the steel frame for SB-208 compliance and corrosion. Unlike others found in the list - corrosion found here will rarely condemn the plane as long as it isn't extensive - it is reparable.
Of course, doing an inspection on your own is of no use unless you know what you are looking for. I'd recommend looking for pictures of corrosion findings on this forum, check out the FAA corrosion control handbook:
https://www.faa.gov/regulations_policies/handbooks_manuals/aircraft/amt_handbook/media/FAA-8083-30_Ch06.pdf
And there's a great article that covers pretty much my entire list above and has some great pictures in the MAPA magazine in October 2012 - I recommend you check it out:
http://www.themooneyflyer.com/issues/MooneyFlyerOctober2012.pdf
BTW - please don't consider me an "expert". I just bought my Mooney in January and am absorbing things like a sponge, but still have tons to learn. I'm sure I'm missing some things. I highly recommend using a MSC for pre-buy inspections if possible.