The M6 does not have a viewfinder, so you may need to budget for one of those.
Have you considered the M50? Although nominally a lower model than the M5, it still has a viewfinder and has the DIGIC 8 processor along with a better AF system (better in low light, more AF points). Can be bought new for £500 so will definitely fit into your second hand budget.
I don't think I could manage without a viewfinder. I find that a large zoom or bright sunshine make it difficult to get photos framed correctly.
A quick glance on ebay says you will only get £60 or so extra for selling as a kit assuming you have the 18-55mm. It depends on whether the extra 10mm are important to you.
Sorry, bit late to the party here. The M series look very nice cameras with very nice end results, but the one thing that stops me buying into this system is the lens lineup, there’s not a single lens that interests me tbh. That’s just my preferences though, hopefully you’ll get on with the M5. You should get great results with any of these cameras tbh.