On the used car market modifications aren't considered to be a plus, rather the cost of restoring the car to it's original condition is subtracted from the price. Of course, to a Volvo enthusiast, the modifications would be desirable, so obviously the seller will do better to sell to a Volvo nut. I'd use the kbb.com or edmunds.com "private party" price as a starting point for negotiations.

I have a T5 5-speed and when I bought it someone (I don't know who, but it's a large church) at my church also had a T5 5-speed. I haven't seen it lately. They do exist. Used 5-speeds do show up with some frequency on e-bay because Volvo dealers can't easily sell them.

If you pursue the ipd car, pay particular attention to the clutch, transmission, engine mounts and cv joints. "Check Engine" light, too. You don't know how misused or abused any other used car might be, either.

-Punxsutawney Phil