I would possibly add that 5 weeks use a year would make your stays quite expensive when you work it all out as a day rate, so as another option consider buying and advertising for summer lets to make a decent income each year till you retire to it. I think it's not a bad idea if you add that to it.
As bewva says, 16 years is a long time, especially in Turkey - but if your plans all seem to end with you retiring to your own apartment, I'd be inclined to buy it now while bargains are to be had and get 16 years of use and income, rather than wait and see what the market is like in 16 years time. With Fethiye, it's only ever going to improve - with promenades, marinas, who knows even big cruises docking daily - if it was Marbella you'd wish you'd bought when it was a sleepy little fishing village and Fethiye is no different in that respect.