Advice please on buying a course measuring wheel

Just get a bike with a well-calibrated cateye or other computer, and ride the courses backwards so you are on the side of the road you would normally run on. They can be calibrated to a 1 cm per revolution accuracy, which is about 0.5% (about 1 in 200), so they should be accurate to within 9 yards per mile.

Yep, I use my bike computer. Just make sure that you're consistent with tire inflation. I check mine against a local half marathon course that has mile marker signs. If it's spot-on over 3 or 4 miles, I know I'm good to go measure. I have an old Nashbar unit that displays hundredths of miles (i.e., about 50 foot resolution).

I have seen some larger diameter wheels at farm supply stores that are reasonably priced.