I'd start buying in essentials just incase. I had my first at 28 weeks and literally had nothing baby wise, and had to be out rushing around to buy everything as soon as they let me out of the hospital. Having to rush around so much so soon after labor is not enjoyable at all, especially if you'rehaving to leave your baby in hospital while you do it.
Don't panic and go buy everything in one go, just try to make sure you get all essentials (bottles if bottle feeding, pram, car seat etc) and then just add little extras as you go along.
I would say start getting a few minor things just in case. It's no mothers dream but some deliver way earlier than planned & not having a thing would be stressful. My bf & I started getting diapers & a couple outfits when we found out the sex, but then waited until the shower(I was about 33weeks) to get everything else.
Am 30weeks and waiting for christmas to start buying things. It all depends on you but some people prefer to be completely prepared by 7months.