My D8 has been working great for me. I would opt for a lower KV motor than the Castle one. If you're short on money, go for a Medusa with a KV between 1600 and 1900 for 5s. And use the calculator that Overdriven linked to to figure out your gearing. Aim for between 30,000 and 35,000 RPM.
Edit: jhautz beat me to it... at least we were thinking the same way.
