Altough there will be a lot of APC recommendations, in my experience they eat through batteries like crazy and are insanely overpriced, especially considering the quality of the converters they use, especially in their smaller/cheaper units.
We needed to replace the batteries in our APC UPSes at least every ~2 years - and those batteries are indeed completely dead and gone after that time! pufeed up, with melted and smored connectors and all - it's absolutely crazy what we pulled out of the APCs even without many (or any) actual power outages/transfers to battery during that time.
We therefore have been replacing the APCs with cyberpower for a while now, and they are MUCH kinder to the batteries (as well as much more economical in initial pricing). I've replaced the batteries of the main UPS in our head office rack last year after a little over 3 years (despite them still being reported as healthy) and most of them are still running in smaller UPSes for clients or e.g. branch network cabinets. This won't be possible with any battery I've pulled from one of our APCs over the last ~10 years.
The price tag of ~100EUR is VERY low (too low) for a UPS for a server - you should really consider setting that bar a bit higher to get someting that's really usable to bridge a few minutes (depending on what kind of outages you are facing) and/or depending on what load you want to put on it. Remember that a shutdown can easily double the normal load of a server (e.g. when shutting down a few VMs), so you should always add plenty of headroom to the average power draw of the system when sizing a UPS, or it will just cut power due to an overload condition (i.e. you've only thrown out money for delaying the outage a few seconds)
In the low-budget range I'd go with something like Blue-/Powerwalker which has relatively decent hardware, considering the price, and they are using standard, off-the-shelf batteries, which are broadly and cheaply available. Of course this only holds true for the "block" type UPSes, not those tiny power strips which are completely useless for anything but a radio clock anyways...
I myself am using a small(ish) Powerwalker VI 1500 RT HID at home for my rack, which I recently upgraded with an external battery pack. I'm living at a rather old farmhouse and we had overhead power lines to the village until ~2 years ago, so power outages/surges during thunderstorms were frequent and due to the ageing cabling and voltages dropping below 210V rather frequently, I'm usually facing at least ~1-2 transfers to boost-mode a week; so the UPS is really working here and the batteries are barely ageing.
If you are fine with some brick on the floor, you can also shed off some bucks, as rackmount usually comes with a premium price, but 100EUR still is quite tough. You also won't get anything with a network interface in this price range, so have a look at
sysutils/nut to shutdown the system(s) you connect to that UPS. It can read out serial/usb as well as informing other hosts via network about the status of the power supply/UPS status.
