Good budget UPS for Synology DS218plus

What purpose will this UPS serve?
Is it just for power conditioning?
How long do you need for the UPS to supply battery power?

I have no idea of the power requirements for the Synology DS218plus , but it does not look to be too demanding.
My suggestion is to go to the APC web site and access their ups selection app.

For more detailed questions, contact APC directly, they know their products.

Maybe it's just me, but it seems there needs to be more clarification between power conditioning, regulation, surge protection, etc.
I notice many use an UPS for the purposes of power conditioning. A UPS will switch over to battery power in the case of an outage. It is my understanding that is it's primary function. Keep in mind, most UPS do not produce a clean sine wave on battery power. The more expensive models do. The higher end models will also provide better protection against surges and transients.
This is something to consider (one example) if you are talking about putting a high power audiophile quality amplifier on an UPS. These components are not designed to run on power normally generated by a switching power supply, like a wall wart. The best for a high quality amp would be to plug into a protection device, but also a device that will accommodate large power excursions when the amp is running a high power levels. It is doubtful an UPS would be applicable for such a job. A power conditioner (ie; Furman, Panamax, etc.) would be more appropriate. As far as the concern about outages and brown-outs, a quality conditioner will REMOVE power when there is a big rise in OR fall in AC power. Under these conditions NO connection is probably the safest connection. Keeping in mind running on AC that is very low, can also cause damage, especially to some motors.

Any component with a hard drive, like a PC or DVR, should be on an UPS. The UPS will give you some time to power down a PC, etc. during a prolonged outage, reducing the risk of drive damage or loss of data.
But when it comes to other components there should be some thought to how each one operates, and if just turning it off (if you are home!) and removing power during an electrical storm might not be more prudent than running it on battery power.
I don't think there is a one stop solution is all cases. Just a thought.
Regulation is another concern. Very few 'power conditioners' also regulate voltage, though some do. The best protection and regulation would likely combine an isolation transformer and a regulator. This is desinged to keep the input AC power operating within a 'safe range' and may have little to do with outages or brown-outs, but for a facility that has less than ideal power at the outlet for various reasons, or you are working with particularly sensitive instruments.