debugging of complex 64b targets and proper tools

WinDBG is definitely the best option in my experience. You should be able to use the WinDbg engine in IDA now I think as well. WinDbg isn't too bad to get use to, and its powerful and scriptable. Debugging services is simple too, you can set your target to execute in WinDbg when the service starts: http://support.microsoft.com/kb/824344