Rent is the number to verify first. The Residential Tenancies Act, 2006 sets what a landlord can and cannot do about rent increases and notice, and a below-market lease you inherit from the current owner can depress the return well below what an empty unit would command.
Suite legality decides whether that rent even counts toward financing. A City of Saskatoon permit is required before a secondary suite's income is credited on a mortgage application, and an unpermitted unit also cannot be properly insured, which is a risk that outlasts the purchase itself.
Ask for the lease, the permit file and the last two years of expenses before you decide, not after. Get your hand-reviewed valuation from Joel Dyck.