I just wrote a detailed post, pressed something on the keyboard by accident and lost the whole thing. Grrr. I'll start again.
We were planning to move to Ireland anyway (DH is Irish) but decided to self build to get as much for our money as possible. We've been here a long time now & are well settled & happy, with a lovely house of course 
We didn't live in a caravan while we built - with small DC I'd have lost it, totally - but rented a house for a couple of years.
It took a while, but we eventually found a site with outline planning permission, and bought it using cash from the sale of our UK house. We then designed the house, applied for planning permission and once that came through got the mortgage, which we drew down in stages as the build went on so didn't have full mortgage repayments until we moved in.
We were fairly sure what we wanted so I drew up a rough design for the house then got an architectural technician to draw up detailed plans for the builder to work from. In some ways, although we saved a lot of money by not using the services of an architect, we might have avoided a couple of design niggles if we'd done so. There are a few things the technician didn't notice that an architect would have done, which we have to live with, but I think there are always things you'd change anyway, you never get a perfect house.
Architects' fees are expensive and are usually worked out as a percentage of the cost of the build, I can't remember exactly how much. Choosing the right architect is important too, you need to chat to a few and make sure the person you use shares your vision and doesn't try to give you what they want, rather than what you want - or as happened to friends of mine charge you a fortune for plans they'd already drawn up for someone else. Picking the wrong one can be an expensive mistake as if you change your mind you will still have to pay them.