Get a UPS anyway. I have a similar setup while living in Mumbai. I got a cheap Luminous UPS. It runs my router, Pi, and Synology for barely 20 minutes, but that's enough time to shut it down gracefully.
We get around 1 power cut every 3 weeks or so (averaging out). If it's the same in your area, then that's like 16 power cuts a year when your HDD is just knocked out. That's too many.
Keep an eye on the temps, if your HDD is constantly running too hot, you might want to improve the cooling.
Also look up hdparm utility, it allows you to put hard drives to sleep when not in use.
Finally, you could consider getting a smart plug and schedule your system to shut down at night so that it's not running for 6-8 hours a day. Then the smart plug switches off. When it switches back on, your Pi will switch on with it.
