The 150 I had pre-buy'ed today has an O-200 with 940 SMOA but this
overhaul happened 24 years ago. Mechanic has pointed out that
Continental also specifies a time limit of 12 years on overhaul and thus
this engine is technically runout.

Do I have much cause for concern?? I know engines are quite regularly
run well beyond that time interval. After all, to make that 1800 hours
in 12 years you have to fly 150 hours/year.

Compression is good (all in the 70s) but there is no oil analysis
available. Engine sounds just fine and also has good oil pressure.

I basically need to make a decision on this quickly and am stuck on this
issue. I am leaning heavily towards buying it. My mechanic has said it
is pretty unlikely that this time factor will result in serious problems
now, but that there's really just no way to tell without an oil analysis
or by watching oil consumption.