The biggest challenge here in West Cork is the endless rain and mud, winter and summer. This makes the hooves soft and encourages thrush and infection in the frogs and heels. It's all very well to show the wonderful hooves of horses in the mid-west of America, but you guys wouldn't believe the mud we have here!
I've been through the process of taking shoes off horses that are used to them, twice. The first month or so they get more and more sore and you really wonder if it is fair to put them through so much discomfort. I would advise riding them in boots from day one, there is no advantage to them getting sore heels and soles. Then they slowly start to harden up but it takes a good six months to get to the point of being able to ride them in a sand arena and occasionally on the road without shoes. But I could never get their hooves hard enough and putting eight lots of boots on to exercise two on the road and going back to look for them when they fall off is just no fun.
I've tried all kinds of supplements and potions for the feet and found none made any noticeable difference. The horse were in good condition anyway, shiny coats and out 24/7/365.
Re the boots. Marquis are goodish but they do fall off, even just trotting on the road. Easyboot Bares are better, but still fall off on the road occasionally breaking the gaiters, and they are black so not easy to find!
There are two major downsides to barefoot for me - chasing after the boots when they fall of and the need for studs when jumping competitively outdoors. So I have reached a compromise. When the horses reach serious competition level aged 5 or 6, the shoes go on in April, with stud holes. And come off again in October for the winter. This works well for me, and keeps the cost down (at €60 a set of shoes). I have had no problems with jumping the younger horses all summer without shoes, no swelling or lameness or splitting of feet or all the other dire consequences of riding without shoes that I have been threatened with.
But if a horse has had shoes for a number of years, don't underestimate the effort and discomfort you will cause the horse in getting him sound barefoot. Get some good boots before you start.