I actually live on this lake so am pretty familiar with it :)
Comps pretty much anywhere on the lake should be adequate, as long as they are waterfront. Most appraisers would even look to neighboring (smaller) lakes for comps if none existed on this lake (I don't agree with this practice)- but this lake has no shortage of waterfront homes for sale, so the 6 comps you have should be pretty representative, adjusting for finishes etc. I think 600k is certainly possible, but 75k rehab may be low. Also depends on what area of the lake and amount of waterfront. North side of lake will command higher selling prices than south. In this price range buyers are definitely looking for a 2 car garage, so I'd factor in that cost. My neighbor has a 4/3, 2 car garage - everything recently updated, North end - move-in ready. 900k - had a 800k cash offer within 1 month. Deal fell through but thats a different story... Also - I'd get a good septic inspection done. New septic system can run 20k+ easily. A common issue around here. Property on this lake is a finite resource, always in demand. Good luck! Sounds like you may be on to something here, if you can get in for the right price.