The most important about equipment is: finding a good supplier. I'm not going to give an internet address, because many things needed cannot be bought online. Oh, they're sold there all right. But for a pair of shoes, a rucksack or rainwear that you keep on daily for two weeks or more, heavily loaded, you need a qualified salesperson. We learned it the hard way, we had to buy a few things twice.
Now we've found a supplier who is a mountaineer himself and who really takes his time to find the right stuff for us. He made Dad try on four rucksacks before he was satisfied, and that choice was absolutely right. Only after that did Mom realize how lucky she had been with her rucksack bought in a sports department store, which fits well, too.
So the most important advice I have is: Test yourself and your equipment on day or weekend trips, preferably so close to home that you can call a friend to pick you up from the roadside if something goes wrong. And don't do it only on fair-weather days, because you are going to encounter foul weather.
One example: I had bought myself a new pair of summer hiking boots, perfectly breathable. They were wonderful, until a thunderstorm drenched us on a day hike. We found shelter in an inn, and I changed my clothes. The next part of the hike went through tigh-high vegetation, so the new pants were soon as wet as the old ones. We were only two hours or so from home, but all the same, I spent the evening in my bed, shivering, feverish, my feet raw from the wet cotton socks.
A few weeks later, with new, watertight shoes and pants, I could walk through an afternoon of downpour with no ill effects.
