401k loan for primary residence duplex

[Jamie Haas]

Found a potential property which is a duplex for 40k. But seller is needing $$ fast (purchased an apartment building that needs work) and 401k loan is the only way I can come up with funds that fast. 

I like the idea of the 10 year term on 401k loan for a primary residence, my question is, has anyone done this and what type of documentation would the 401k administrator require to prove it's for a primary?

I would pay off in march with a long term compensation that comes due.