ONKYO 805 vs 875:
Also, I don't really have a set limit on how much I would like to put in. I was thinking of getting a av reciever and two front speakers for now? and build it from there.
The biggest difference to me is that the Onkyo 875 is 140 wpc, has 1080p Video Upscaler, 4 HDMI inputs, & costs $1,300. The 805 is 130 wpc, no 1080p Upscaler, 3 HDMI inputs, & costs $900. Is the Onkyo-875 worth the $400 difference? I don't think so IMO.
You definitely need to audition speakers, preferably in your own home (free return policy if don't like). Only you can judge whether a pair of speakers are good or bad.
It is very true that, like all things, speaker Frequency Response (FR) in general are only a tiny part of the story. It is also true, however, that excellent speakers will have excellent FR.
I recommend the Definitive Technology BP10B tower speakers for $900/pair. They are 42" tall, high quality build, & are Bipolar design. Their F.R. is 20Hz-20kHz +/-6dB & 25Hz-20kHz +/-3dB (90dB Reference) according to Julian Hirsch & Stereo Review Lab. They are 6 ohms speakers, & either Onkyo models will play them just fine.