I'm probably going to wait until quite late on just because of my last pregnancy really. I didn't start buying in my last pregnancy until 22 weeks, had the stuff 8 weeks and then had to send it all back. I'm so glad I kept all tags on etc didn't take pram outside or anythin like that so I could send it all back. We had everything pretty much by 30 weeks. Tbh I've go most things we need esp if we have a boy as I have tons of baby gros! Kept the new cot mattress we bought, just didn't get around to sending it back and it was a bargain anyway. I still have a new Avent steriliser and bottles, still got a couple of packs of nappies too! Only really need a pram/car seat/changing bag, bits of clothes and bedding. Oh and could do with a breast pump!
The way I see it really is that things could go wrong at any stage in pregnancy and you have to remain positive or it kills you. I think you just have to enjoy it and go with what feels right for you. All the shops I had bought stuff from were lovely and didn't even question me just told me to bring straight back for a full refund, mamas and papas were amazing, very professional and sorted everything quickly for us.