Discount on price due to existing lease?

[Tim L.]

I'm currently looking at a property that is rented out to a tenant, with the lease ending March 2015.  The rent is below market ($1050 when you could easily get $1200), due to the fact that the current owner knows the tenants personally (current owner is tenant's boss).  I was wondering if the owner would be willing to take a discount on the price due to the fact that we won't be able to move into it right away, and because we haven't vetted the tenants whatsoever.

The market in my area is hot right now, but I think that since I'm taking a risk with an unknown tenant, this should be reflected in my offer price.  Do you agree?  If so, what % would you use to account for this risk?