I currently own a sonos play : 5 (gen 1), and I'm planning on building a 5.1 surround sound system with a budget of about 650-700$. I was thinking of buying an used 4k receiver (about 250-300$), a pair of these
as front speakers and these as rear
and then using the sonos play : 5 as a center speaker using a speaker to aux cable.
And I already own a subwoofer from sweden that I don't know the name of, but it's a 15".
Is this a good idea or should i rethink it?

May 30, 2018
I went with all Fluance in that price range, except for the subwoofer but you have one already. SX6 front channels, XL7C center channel which I'm still using, and AVBP2 surround channels. Very good for the price.
For the front 3 channels at least you will want timbre matched (aka from the same company and speaker line) or you may have disjointed panning from left to right and vice versa. It will also greatly impact multichannel music if you enjoy that sort of thing. The refurb Yamaha's massdrop has been offering are a good deal and I would choose from whichever of these speaker lines work for you: Pioneer Andrew Jones line (has towers, bookshelves, centers, subs and atmos speakers) Elac Debut Series also Andrew Jones (same line up but a step up in class) Emotiva Airmotiv line, no atmos speakers yet but those matter the least on the matching front
Another good source for refurbs: They typically come with a factory warranty from here, double check each brand though...
May 10, 2018
I'd go with the ELAC Debut speakers by Andrew Jones. They're in the same price range but an order of magnitude better in terms of sound quality.