If you absolutely have to go Novak then go with the 7.5 but be reminded that
it can only do up to 4S or 14.8V maximum input.
If it were me it would be the Mamba Monster Max/2650kv for 4S or the
MMM/2200kv for 4S-6S. I believe this to be an overall superior system.
That being said, its always a good idea to wait for others to chime in and
take note of their advice.
Here's a link to the guy who undertook a very successful E Slayer Conversion.
Hope this helped.