Here's what I mean. I'd rent all the mission-critical stuffthe regulator and tank in particular. Probably the buoyancy compensator, or BCD, as well. The reason? For one thing, this stuff is bulky and heavy and will make getting through airports a pain. For another, if you get it all over there and then have some sort of mechanical problem, you'll end up renting gear anyway. If you rent gear and need to swap something out, then that's not a problem. Plus, you might be able to try several brands of equipment, or different styles, and that will help you make a better buying decision when you do invest in your own gear.
In contrast, I'd be somewhat inclined to take all the "comfort" gearmask and snorkel, fins, diving suit if needed. Sure, that stuff will be available in Turkey, too, but it's not hard to pack. Plus, it's stuff where fit and personal preference may make a difference in how much you enjoy the divesmaybe a particular kind of mask fits you best, or certain fins give you a favored propulsion level. And of course, if you have an underwater camera or anything, take that along.