I agree with the 8 hp/ft recommendation.  I had a 15' JD 726 finisher that I pulled with a Case 2096, dynoed at 125 hp.  It did ok, but you wouldn't want anything less on there.  My ground was heavy clay, so it pulled pretty hard.  If your soil is more loamy, it should be a little easier pulling.  I would guess that the finisher you're looking at would be similar to the JD I had.  You definitely need to keep the speed up to 4.5-5 mph to do the job, anything less won't break things up and level good - 6 mph is better. 

HTH,

Mike