I have the chance to buy a 1991 Yamaha Grand G2 with disklavier for $8K. It plays beautiful and it looks excellent. Now I don't know if this price is too high or I should wait and aim for a different piano. A lady is selling a 1972 Steinway model M for $12K (I might be able to get it for around $10K), whick looks pretty good, but it probably needs some reconditioning. I hasn't been tuned in years and the walnut finish is a little faded on the top, but overall looks good and sounds good.
Now... I'm looking for a piano that will last me forever as my girls are little and they are starting piano lessons. I have no real knowledge on pianos only what I've read and what I've gathered from other peoples opinions and experiences. Which one would you buy?

You should have a tech look at both pianos and advise you.

Bob's succinct advice is excellent. Any 37 year old piano which has been neglected will need attention and there could be problems with the other piano as well. Only a qualified, independent technician can give you a thorough assessment of both instruments with valuations and estimates of required re-conditioning.

Best wishes for your decision!

Robert.

As both Bobs stated, both pianos need to assessed by a qualified tech for you to know exactly what you dealing with.
Actually both pianos are good $ values if they pass the scrutiny of the tech though they are totally an apple and an orange.
A 1991 DG2 Disklavier is a "wagon grand".I would do a search on "Disklavier" and "wagon grand" in prior recent threads in this forum. I don't have the energy to retype the pros and cons of the older "wagon grand disklavier" again.
A 1991 Yamaha G2 in pristine condition can be considered a newer piano though to many the disklavier is considered obsolete compared to the modern player systems of today.It's all in what your expectations are of this 18 year old computer.

As for the 1972 Steinway M, my prior assessment in your alternate thread is incorporated by reference to this thread. I used to be a lawyer in a previous life.