This is an old thread, but as there have been no replies I thought I would comment.
I'm not a financial adviser and have no affiliations to any financial institutions.
HSBC Turkey do provide loans secured on property in Turkey. Perhaps other banks do as well but I'm only aware of this one.
However, there are some considerations to be made. The maximum loan is 65% of the bank's valuation of the property. From experience this valuation will be considerably less than the market value, as it would reflect what they could hope to get for it in the event of a default and a forced sale.
The period of the loan can be from 6 months to 10 years, and the rate of interest rises according to the length of term of the loan. Currently it is between 1.25% and 1.55% per month, which is an expensive way to borrow money.
For mortgages on new properties, I know that banks will not take into account any potential earnings from rentals, they are only interested in current employment.
For this type of loan however it may be different, but even if it is, the borrower would need to be able to prove the rental income, which would mean producing tax returns showing what has been declared.
For anyone who may be interested, here is a link to the relevant page on the HSBC website where more detail is provided:
HSBC Personal Finance Loan
