600 watts ~ 1kVA
You will get a good 1kVA UPS for 8500. The 'truck' battery which is 195amp will cost you somewhere around 10000 depending on which one you get. As per your requirements of 2 PC's, one fan and light, you most probably will get 2-3 hours backup with that. I am basing this on my findings. I was running a P4 with the 1kVA UPS and with a dry battery of 60amp and it was giving me 2 hours backup. Since you have 2 pc's, a 120amp battery should give you the same. And since you will be running a tube light and a fan (fan takes around 70 watts) as well, a 195amp battery will be just right for you.
However, the biggest flaw with this whole UPS system is that you can never be sure using mathematical calculations. You have to try it to find out. So i would strongly suggest find out from a shop who seems like a knowledgeable guy.