Personally owning 4 UPS's I can safely say that even the cheap UPS's do their job fairly well. The most important thing you have to consider is the load you will be placing on it and what sort of run time you want from it. A typical desktop turned into a server will draw around 200w at most, so a 500VA (425w) and higher should do the job just fine. However you'll find the cheaper units contain a single small battery which is enough for just 5-10 minutes uptime, less at full load. If this is enough and all you want to do is cleanly shut down then these should be fine. If you want a longer sustained uptime then you're looking at more expensive units with extended runtime boxes for them.
I have a 1200VA UPS with 2x7Ah 12v batteries and it runs my server for 1 hour (it draws about 150w) which is plenty. I did however do my research before buying as I wanted that dual battery setup.
If you want quality, definitely go APC. They have a good UPS selector wizard on their site, but you'll have to be prepared to pay the high prices. Since you don't have an expensive front facing server I wouldn't expect you would go for a high end UPS either.
Hope this helps.