Is this model suitable for Canon 750d?
Depends on your definition of suitable. It will work on a 750D and won't hurt the camera.
Is this flash good?
It does what it's designed to do, and is decent value for the money if you're a hobbyist and planning on occasional usage; for a pro doing hard daily usage, maybe not so much. The main headline features are that it has a radio receiver built in that lets you control the power and zoom of the flash remotely if you trigger it with a YN-560-TX or YN-560IV on camera.
Is there any better model?
There are many models that have better feature sets and reliability but they'll be more expensive. At the same price, personally, I'd say the Godox TT600 is probably a better value at the moment, but that could change any day because gear at this level turns over fast—new models appear all the time. The cheap Chinese side of lights is a continually shifting landscape.
The Godox TT600 is also a manual-only speedlight with a built-in radio receiver, only the Godox X transmitters can remotely control the power setting and give HSS capability. But more importantly, the Godox triggers can do TTL (automatic power setting based on metering) and HSS (high-speed sync—the ability to use a shutter speed faster than 1/250s with flash), and Godox is building them into all their current flashes, both manual and TTL, speedlights, bare bulb flashes, and studio strobes. There's a lot more room for expansion than with the Yongnuo system. The X triggers also come in Canon, Nikon, Pentax, Sony, Olympus/Panasonic, and Fuji flavors, and any lights with an X trigger built-in can switch between the systems. Yongnuo only comes in Canon and Nikon and Yongnuo lights don't system-switch.
What is the difference between YN560III and YN560 IV?
The difference between the III and IV is that the IV can be used as a radio master as well as a slave. Unlike the YN-560-TX, however, it can only control three groups.
Do I need to purchase triggers to control remotely these flashes? If so, what triggers should I buy?
Yes, you need an RF master unit on the camera hotshoe to fire these flashes off camera. And the only triggers you can buy that work with the built-in receivers in the 560s are the RF-603/603II/605 and the YN-560-TX. And those triggers do not work with Yongnuo's YN-622 or RT triggers. So if you decide later that the limits of a manual-only flash are bugging you and you want to upgrade, you'll have to rebuy everything. Which is why I think a Godox TT600/X transmitter combo is better value.
BUT. I'd also say that for a first or only flash, get one that has TTL/HSS capability, not a YN-560; and if you care about warranty service, future compatibility, and resale value, an OEM unit (e.g., 430EX III-RT). Because a YN-560 is a manual-only flash, it has a lot of limitations upon use, both on- and off-camera. To me, manual-only cheapies are great as 2nd, 3rd, and 4th flashes for studio-style off-camera setups. But for on-camera bouncing with run'n'gun event shooting, TTL makes life a lot easier, and having HSS capability can be nice for thin DOF in bright sunlight.
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