I've installed KDEPIM 4.6 and kmail2 works fine The email accounts are now resources in Akondi but I have no idea how to get them indexed and for that map it's not particularly obvious have to search the email when I do get it indexed.

Email indexing should be enabled by default. Make sure you have Nepomuk enabled and it should happen automatically. Strigi is not required.

It seem everything didn't get installed initially, when I installed akonadiconsole it pulled in more stuff. It works now.

Is there a way to adjust the point it stops indexing due to load?

Unfortunately i'm not aware of any setting which allows that to be tuned at this time. I know Strigi won't index when the system is busy or running on battery power.