Buy boots first. Go to somewhere like Cotswold Camping and get your feet measured. Take your time, dont hurry to make a decision. Personally I would take a few full day walks well beforehand if you can, you may like it, but you may also realise that walking long distance is not for you?
If I was starting all over again I would buy 'cheap' boots first, just to be sure that long distances is what I want or not. For example, I bought a Decathlon pair of boots recently and then walked 140 miles through the Pyrenees. I walked on all surfaces, grass, dirt, gravel, rocks, tarmac etc and they were fine (they leaked on a rainy day but they dried quickly), they were also very comfortable, easy to lace and when I look at the soles very little wear seems to have occurred.
The one thing I've discovered which suits me fine, is that I always have a Sorbothane insert in my boots, (the other brands I'm sure are just as good) on top of, or underneath the normal boot insert. At the end of a long hiking day my feet sometimes swell a bit and my boots can become too tight for comfort. When this happens I just take out the normal boot insert which allows my feet to then fit into a slightly bigger space so that I can walk easy again.
As for the rest of your gear, well for me thats when it can be a bit more complicated. I'll leave it to others to suggest stuff because I'm just out to hike a 10 miler! Its a lovely day here and I don't want to miss it.