Ace,
(none of what follows is a subsitute or relief from checklists, or normal procedures)
When you fly the prospective 150, take the opportunity to do your own extra thorough walk around inspection. Have a good look at the following for starters: Structure generally, for any wrinkled skins, or "smoking" rivets (dark stains around them), dents, cracks, missing fasteners, wrong fasteners. Examine the tail tiedown ring, and the securuty of the fitting which holds in into the lower fuselage skin for evidence that it has hit the ground - is the fitting (a welded steel part) loose in the fuselage? The 150D has an odd tail arrangement compared to other 150's. Look carefully in the area of the attachement of the horizontal stabilizer to the fuselage. In particular, the local appearance of the leading edge skin of the stab as it attaches to the main stab skin, at the fuselage - is it wrinkled? If it is, or there are smoking rivets, you might have a cracked rib there. Using great care, press up and down on the tip of the stabilizer only enough to just change the position of the fuselage realitive to the ground (do not push harder than that). Is there any sqeaking or "oil canning"? I little oil canning is not unknown, but warrants further inspection. Examine the top wing skins just outboard of the fuel tank covers for any wrinkes. Examine the fuselage skin in the area of the main gear leg attachements for any wrinkles in the skin. Consider the reason for any patches you might see while you're looking. When you are doing this, you are intending to look at the entire external surface of the aircraft. This should take you 10-20 minutes. Also check the security and operation of the nosewheel. grab it and pull and push in all directions - loose? Steer it. with weight on it, the centering cam will prevent the rudder control system from moving, so only the lower nosewheel steering will move. There should be absolutely no freeplay between the steering collar (with the two pushrods going up and back) to the nose wheel fork. Nothing should wiggle. Anything (including the shimmy damper) which wiggles will create the possibility of nosewheel shimmy on the ground. Write down any defects you see. they might not disqualify the aircraft, but you want the inspector you hire next to know you found them, and explain thier significance (cost) to you.
As you enter the cabin, move the pilot's seat it's extreme travel range. Does it move freely? Inspect the portion of the seat rail which is exposed. Are the locking pin holes worn? are there any cracks in the top of the rail (between the holes and the edges)? (you'll have to wipe the rails to inspect). Do the controls move freely? Pull the rudder pedal arms toward you on each side (four total). is there a lot of wiggle in the horizontal rudder bars? While you're under the panel, look up, anything dangling or out of place up there? Is there a CO detector? does it look recently installed? Does it indicate CO?
Check the operation of the starter. It probably still has a "pull clutch" (you have to pull the starter handle toward you about three inches to start the engine). If properly operational, these are excellent for reliability, and low long term cost. With the master off (and the area of the prop confirmed to be clear) does it pull and retract freely? When you turn on the master switch, does the turn co-ordinator have a scary load bearing sound? When you pull the starter handle to starter the engine, does the clutch engage smootly? The stater motor is not energized until the clutch is correctly engaged? (no grinding gear sounds?) Does the enigne start and respond to the throttle well?
Go flying: Does it taxi straight if not steered? Brakes work effectively and evenly left to right? Not soft? While you are taxiing, deliberately find some uneven ground to taxi over. While crossing it, carefully watch the loer edge of the windshield relative to the place where it meets the glare shield. Does the winshield shift left to right relative to the glareshield as the airplane goes over the bumps? In the air, does it fly straight in smooth air? Using suitible practices, stall the plane with the ball right in the middle the whole time. At the stall, does it drop a wing? If you do it again and again, does it drop the same wing each time? Within appropriate operating limits, operate everything (except the fuel valve, Mags, and mixture idle cut off!), does it do what it should?
During landing did it shimmy or pull to one side. After you park, but before shut down, at enigne idle speed live mag check, by turning the key switch all the way off, then on again. did the engine turn off while still turning? (If not, do not touch the prop of this plane for any reason - see later check) Turn the fuel valve off, did the engine begin to stop after about a minute? Don't let it stop yet, turn the fuel back on. At 1200 RPM, pull the mixture out, watch the tach carefully; did the RPM increase a tiny amount before the engine stopped? Once the engine stops, are there any noises from the gyro instruments?
Once out of the aircraft, and it is tied or chocked, and the engine has cooled for five minutes, confirm the mags are turned off, key out, mixture idle cutoff, propeller area clear: Pull the prop through without allowing any part of your (or anyone else's body to enter the prop arc). Do eac of the four strokes seem the same as each other? The prop should seem to "bounce back" each time the prop stops on a compression stroke. These bounces should be more or less the same as each other. If one or more has no bounce back, you probably have a "flat cylinder" $1500.
You've spent at least an hour and a half with the plane now, and have a good sense of it's condition from a pilot's perspective. Any defects you have noticed are noted, for your inspector to consider. One of two failures in the foregoing, are not a "no" right off, but could be an indication of some cost to come.
An inspector will need a full day with the plane, but should not need much more. Having the logbooks in advance will be a help to him.
There are many more things to check, but the foregoing will give you a good idea, and if there are a lot of "failures" of your inspection, consider not going further with the purchse, or negotiating a price reduction, at least to cover the full cost of a very detailed (two - three day long) full inspection.
I'm sure others here can offer their advice too, but this will get you started. Good luck....
Pilot DAR