what kind of king air time c-90 or 200?
As both are single pilot aircraft that time will not count toward your 500 multi crew time for your ATPL.
You must have an IR to do that course and be current if you were not already aware.
Also a FI(R) is a very handy qualifcation to have (possibly can help you in the future in an airline check and training situation). Thats fairly biased as I come from a instructing background, main benefits is you can get a lot of TT (S/E i know) but in a very short time frame, also a I/R does alot for your own development and job prospects.