Ace,
A thorough corrosion inspection will take a minimum of 2 working days, but probably more. In particular, the fuel tanks should be pulled to look under and around them, and that is a misreable job if they have not been out for a while - could be a whole extra day in itself.
The problem is, the other guy owns the plane, you're paying to have it inspected more thoroughly than it proabaly has been in recent times, and the inspector finds something which renders it unairworthy right where it is. The present owner will not be really happy about that. It will be his problem though, not yours. There is no way of knowing what you're going to find. It could be just the one spot, but more likely there is more waiting to be found. 25 years ago I was asked to ferry back a 150, which had not flown for a few years. During my preflight, I found corrosion which I did not like, on the fuselage near the wing. I did not fly it, and after inspection, it never flew again, it was trailered away, and parted out for its pieces.
The other way to approach this plane, is to consider how long is left on the annual inspection, assure it is simply safe for the year's flying, and fly the heck out of it up to the next inspection date, with no expectation of it passing. If it will, yippee, if not, part it out. The parts have value in the market. You will loose money, but if you have several hundred hours of flying under your belt, on your terms, it was probably worth it. Worst case, is that something goes wrong and grounds it sooner than you plan. It could happen, then you've got decisions to make. Either part it out right then, or perform little fixes, just to keep it going, but they will add up fast! I flew many hours on a number of planes, then to fly them to the bone yard for their last flight when their annuals expired. The most we ever put on one was 80 or so hours. In 5 or 6 planes, we never had any maintenance required for safety, during that period. Our record was good, and I'd do it again (other than I own a 150 now!).
Will the seller sell it with a fresh annual inspection on it?
Whatever the case, you want a plane which you know is at least safe to fly. If you can't be assured of that, walk away. A corroded aileron is not likely a safety concern, unless the corrosion is compromising the hinge, control link fitting, or balance weight attachment. Ailerons can be repalced with used airworthy parts.
Investing $20K in this plane is a poor idea. This plane is only worth the cost to keep in safe and legal condition. There are lots of more recent 150's (as well as all kinds of other aircraft) in better condition, which would be much more valuable than this one, after you'd spent the same money to restore them.
Another option, is to offer to rent block time privately from this owner. Commit to 100 or 200 hours in the year, but only pay in 20-50 hour increments, with the understanding that he has to keep the plane airworthy, or you won't pay for any more time. At the end, you walk away with the experience, but not the problems. Maybe, if the present owner sees a revenue stream, he will hold onto it - depends upon his reason for wanting to sell (does he know something he is not telling you?).
It is not possible to have a "no risk" aircraft ownership experience, all you can do is your best to minimze expensive suprises.
In no case should you consider buying this aircraft without an independant inspection, and a good talk with that inspector afterward about what he recommends. His knowing what you are trying to accomplish will probably help him help you best.
Pilot DAR