Buy recommendation, "back-ups" or new battiers for old "smart ups"?

[BillP]

My Old USV is now broken. ( Model: SUA1000l , APC Smart-UPS 1000VA)

Every time I turn it on it first looks as everything is OK. Yellow LED lights, then the battery-indicator comes to life, it "hums", then all of the sudden: RAPIDLY degrading the shown battery capacity, till only 2 LEDs are left, than the dang thing starts to beep constantly and won't calm itself.

If I turn it off and on several times it works again and shows the battery-charging (even up to full).

So I suppose the batteries are dead and sometimes I'm just lucky it stops noticing...?

Now my question: Should I just buy new batteries (~40-60€ on eBay any tipps on what to look for?), or downgrade to a "back-ups" (~60-80€), or a "back-ups pro" (~120€, but has only 3 powered C13 outlets)? A new smart system is definitely out of my budget right now...

I use it mainly to protect my computer and audio gear and always hoped it would feed "straighter" sinus-power to my active speakers (which it unfortunately doesn't, or at least they still humm when the dishwasher is turned on)...

Edit:
Do the small "Back-ups" have a Ground-screw on the outside? (tvss-gnd)