It's worth what you are willing to pay for it....
The R71 is a 17'-1" boat with a 6" built in setback, so you will have 16 and half feet of boat to work with.
The Mercury 125 is the best motor for that particular boat, since it's max HP is 130 and is a great combination. The R series was a lower cost alternative to the 300/400 series boats and should be priced accordingly. At $8,000, it might be a bit overpriced, but if it's at a dealer and the boat is in excellent shape with all gauges working, all plumbing working, new batteries all around, then $8,000 could be fair.....