Using 600D and looking for flash. My budget is ~RM600. Currently found Nissin Di600 and Di700 are quite good. Did some researches and found 430 EX II is highly recommended, but it is out of my budget..

Is there any better flash out there for such budget? I need one with AF light to assist in focusing.

You can check out the Yong Nuo Flashes. Yong Nuos are seen as some of the best budget 3rd party alternatives for flash guns. The YN 568EX II is the best they got and matches you budget. HSS, ETTL, GN58 (Same power as Canon's 580EX) with AF assist beam. Just make sure you get the Canon version and not the Nikon version otherwise it might not work on your Canon camera.

Or you can look for the Canon 430EX II 2nd hand. 2nd hand matches your budget.

The Di700 has higher power..
And how long does a flash usually last?

Just a quick google and the Di700 has GN54. More than enough power if you want Nissin.

How long a lasting a flash is I guess its luck. But no need to worry about it, as by the time you about to break your flash's mileage limits, you are also about to finish your camera's shutter count mileage limits too!