I'm working on a Raspberry Pi 4 NAS server and I found a power supply (5v 3A) on aliexpress:
Is the power supply going to be capable of supplying my NAS server, if not, what should I be looking for?
Your cheapie power supply looks OK. You might like to read the following article just to make sure.
How much power does the Pi4B use? Power Measurements - RasPi.TV 2019jun25
In case you are considering UPS, here are some references:
Raspi UPS HAT Board
Of course you can consider the official power supply. But they are much more expensive：
Testing Official Rpi4 PSU
I don't like wall warts, so I am using this desktop version:
XiaoMi CDQ06ZM PD 60W 6-Port USB-C Desktop Charger (18W PD Port x 1, 18W QC Port x 1 and 2.4A Ports x 4)
And this discussion is good.
Choosing a Pi4 power supply - 2019jun
I don't know how power hungry is you NAS drive. You might like to read my answer to the followng question to get a rough idea of the problems with HDD and SSD.
Can the 3b+ power two usb SSD drives?
Actually QC3.0 Charger is usually OK for Rpi4B.
Can I power 2 raspberry pi 3 B+ using a USB hub?