I was planning to buy APC 650VX UPS and for that I was going for a Corsair VX450 but from I see I need to buy a none/Passive PFC based PSU for Stepped Sine-Wave UPS . Hence, APC 650VX (Stepped-Sine-Wave) does not fit with Corsair VX450 (Active PFC).
Any advice on buying good quality none/passive PFC PSU ? (also, the UPS is fine ?)
There are no non-active PFC PSUs that I would consider buying today. If you absolutely must though, check out offerings by FSP. Be advised that many of those older models are being advertised at their PEAK rating, not continuous; so a unit they advertise as "400W" may really only be rated for 350W continuous.
If the stepped sine wave is reasonably close, it will still work with many PSUs, although possibly not with the Delta-built Antecs (I had to get a SUA750 for my SG-650). Check with the PSU vendor's tech support or on their forums.