Welcome, Greg! All good advice above, especially about expanding your opportunities. Don't fall in love with the first airplane you look at, especially at this asking price (not exorbitant for a solid 150, but on the high end of the scale in this economy).
Work out who pays for what with the owner before the annual. You of course pay the mechanic of your choice for their concurrent pre-buy inspection (guessing $150-$200... they aren't signing anything off). The owner pays for the annual itself and any needed repairs to airworthiness items (it's still his airplane until sold). Non-airworthiness or cosmetic items do not affect the annual sign-off and are negotiable between you as a condition of sale, or not.
If you like the mechanic that is doing this annual, there's probably no reason not to use him/her in the future. But, you still need someone impartial and experienced on 150's to look over their shoulder for the pre-buy. LOTS of things can be missed that might bite you at next annual with a new to this airplane mechanic.
Your first annual with a new mechanic will probably be the most in-depth inspection it will have for some years to come, and therefore often that much more costly. It's not a bad thing to discover what's really needed, but better to find them now, not after you've bought the plane.
