But the case offers a bottom-PSU socket... is there going to be a problem if I place the PSU upside down so the fan could face upward ? ('cause reading comments, many ppl say the fan is facing downward...
More aboutfacing side potential problem
No that shouldn't be a problem.
the fans on PSU's such as that one, pull air from case, and exit it out of case. air(most likely off your GPU, and out of the machine)
If the power cord out of your psu is really shot and don't reach to top of motherboard where the power connector is, it could be a problem. Otherwise it'll work fine.
Looks like the case is set up to mount a PSU the "normal" way or upside down. Should bolt right in, as for affecting the power supply, it won't adversely affect it. Only thing moving in a PSU is the fan, it doesn't care where it's pointed.