You should be able to add ANY comp to the sales grid and it should adjust to exactly the same value as the other comps. That's the math. In fact, I can put an unlimited number of comps in my sales grid and get them to all adjust to exactly the same value. Which comps I put in the sales grid - are those comps that are most similar to the subject, because if they can be adjusted without running up the gross adjustment totals, then that is convincing evidence for even clients who don't understand the mechanics of adjustments very well. If your client wants some arbitrary comp in the sales grid - it doesn't matter really - at least if you know what you are doing, because if you know what you are doing, it will indeed adjust to the same adjusted sale price as the other comps - so there is no need for averaging or weighting.
My comps can go back 20 years. -- and they will all adjust to the same adjusted sale price. Yep.