If you dont want to replace the battery have a look at the software and see if you can change the "battery installed date" or whatever its called on yours.

my question is: do I need UPS at all?

Depends on your circumstanses.Do you have high priority mission critical hardware/software or need to keep a system live no matter what?
If not then you could manage without provided the voltage to your property is stable and not prone to brown/black outs [short/long interuptions].

I can use $5 surge protector

My opinion for what its worth are that these have a placebo effect and you would get more benefit from the $5 spending it on a beer or two