Ace,
Some of what you have pictured is serious, some is not a big deal.
The attitude indicator, and oil under the engine may be associated problems, as the aircraft might have a "wet" (oil) vacuum pump. The oil separator for these can fail, or have poor hose connections, and leak oil. Not serious. Wet pumps are excellent, when working well, but this one, or the separator might be in trouble. The attitude indicator might be original, and if so, should not be consdiered repairable. Bear in mind, that aircraft is not required to have an attitude indicator, so you could just have it removed. The vacuum pump will have to work for the DG though. (If the plane has instead, venturi tubes on the side, it's a different situation).
The ailerons are normal and fine. The red fairings on the H stab do that all the time, no problem. Greasy flap rollers look messy, but are probably fine. Changing them is not a big deal if they are seized.
The dents in the belly skin ahead of the main gear legs are no problem
So far, nothing is a no go, though do do have some maintenance cost ahead of you already....
As for the oily nose strut, I would do the following: Remove the cowls, and clean the engine, inside of the cowls, and nose strut/wheel very well. Once clean, let it dry while you have lunch, and go back and have a good look around in there, and take some photos. Telling you what to look for in there is beyond the scope of a post here. Once clean and dry, put the cowls back on, and go fly it for 30 minutes. Land back, and as soon as you can, get the cowls back off, and look to see where there is fresh oil leaking. It could be serious, or meaningless, and you won't know which, 'till you know where the oil is coming from. The loss of oil in and of itself, is not a big deal, as long as you always have enough (the O-200 will not be hurt down to 3 Qts, as long as you don't overheat it). If you can see where the oil is leaking from, post a photo of that place here. It may be something you just learn to live with, many do!
The corrosion is alarming. What you have pictured, can probably be cleaned up, and repainted adequately, but it would really concern me that there is more, in more crtitcal places, which you are not seeing. a very thorough internal inspection of this plane is a must. U.S. built 150's never had internal corrosion protection, so it can be a problem. Where you see corrosion around rivet heads, it could be local, or much worse, that can be the final stages of internal corrosion coming out! If corrosion like that on the aileron is found inside the wings, or fuselage, this plane is junk, walk away. Other places, like wing struts, can and must be replaced. There are lots of used serviceable parts out there.
The wrinkle on the underside of the aft tailboom could be pretty serious. Not so much for itself, but more for what else got damaged putting it there. The tails of 150's tend to be a little delicate. whatever event put that wrinkle might have done more damage elsewhere. The corrosion inspection, which is a must, should also turn up any other associated damage in the tail if it is there. Replacing that skin is a big job. If I owned that plane though, I would consider a structural repair involving an internal stiffener there.
I do completely agree with Englishal, if the owner will take $8000 or even $10000 (on the ** conditons), and there is no more serious corrosion than what you have seen thus far, it's a bargain. it will never be a cream puff ever again, but if you fly it 500 hours, and sell it for $6000 (even parted out as a non-flying pane) you still did okay! This would not be a plane I would invest in hull insurance for.
Let us know how you make out...
Pilot DAR