Cheap PSU's are cheap for a number of bad reasons, stay away from them. If you're selecting a new PSU in the future remember that it's better to go for a lower Wattage unit of high quality than a cheap 'high Wattage' unit of low quality, cheap units can, will, and have damaged entire systems.
Yes, you are correct, the R9 270X and the GTX970 draw similar amounts of juice, and with the installed unit having enough PCI-E leads I see no reason to swap it for your planned graphics upgrade (and certainly no reason to inflict that Apevia thing on you system).
Overclocking is fine and safe, provided it's done carefully. Do it carelessly, though and it's possible to kill a system in very short order. I note from the HP site the Kaili MB is a Z97 part, which SHOULD make it overclock friendly if you choose to look into that at a later date...Later
From where I'm sitting, the only possible question is : Will your planned upgrade fit the case? Again, the HP website shows the Kaili to me microATX, so I'm guessing a small case which may, possibly limit the upgrade options.