It looks like you have a decent budget set aside which is good.
Honestly, though, if you are going to spend this much I personally would wait a few more months if you can (Sep, Oct, Nov ???) to see what the new GPUs and CPUs are going to offer. On the downside, of course, you might have to wait a bit longer than this for stocks to get up to speed so I suppose it really depends on how long you really want to wait.
As to where to buy it from you have loads of options nowadays. I bought my PC years ago from CCL computers (not sure what they are like now). Other names to throw into the pot might be maybe Cyberpower PC in the UK or Overclockers. But please don’t take these necessarily as recommendations just ideas for you to follow up. It’s always best to do your own due diligence and look at current reviews, after sales service, warranties, ease and cost of returns etc.
Just my thoughts for what they are worth 