mim- I agree with Duracell. It's not realistic to expect to plane in just 8-12 knots with an 8.5 at your weight, regardless of what board you are riding. Check this spreadsheet to see what sail size you really need to plane in your target wind strength, and then buy a board that can support that size of sail (probably Carve / GO / Futura 144 or larger).
It says you will need 14 knots to plane nicely with an 8.5, and 17 knots to plane nicely with a 7.0.
To plane in 8-12 knots (assuming average 10 knots) you will probably need a 12.0 and formula board.