Can't help you much with the accuracy issue with round balls Pioneer. But the Parker Hales have a good reputation for quality of construction and accuracy with Minnie Balls. It might shoot round balls quite well.

The price seems OK if the gun is in good shape. But I would shoot for $400 (or less ). A replacement military style steel rod will cost something like $40 if you can't make your own.

My thinking would be that I would buy it to play around with for a while. If you decide not to keep it I think you could sell it quite easily (possibly with a profit even) on the North-South Skirmish Association's classified ads forum.