2nd hand pc's are not bad, but just some advice;
1. save ur 15K, then save more until you could come up with better amount, then buy a brand new system, just make sure to check the specs.
2. you could build your own system, compare prices of the hardwares you want to be included.
remember, most people who buys 2nd hand computers may later encounter problems. i have once a 2nd hand pc, i keep replacing the power supply, the hard disc, the RAM and it cost me more. lesson learned.