Having been through a lot of pain ...
From our experience ...
1. The Architect has the 'aesthetic vision' - it certainly does make sense to employ one, but in my experience there are a few other things that you can and should be thinking about at this stage (because that will inform the dialogue you will have with prospective architects).
2. It was the selection of the Structural Engineer that caused us the biggest problem. We went with the a local one (delays) then another local one (more delays, more expense). Ultimately, we took advice from people on this site and used a SE may others have used and the experience has been good. (Hilliard from TSD). As a novice, you think 'how hard can the Engineering be' and it turns out that every SE comes up with a different scheme, with wildly differing numbers of piles required (for example). Also the Architect you select might prefer to work with their own SE (who maybe good or they may not. So you might want to say to the Architect 'are you willing to work with the SE we select?"). Look, I do accept its a bit chicken and egg - but if we had selected our SE early on we would have saved two years!
3. The other question you need to consider before selecting the Architect is how you are going to build the bloody thing. Again, speaking from experince: We made this false assumption that there is a 'market' in building contractors, and that when you put your designs out to tender, someone will give you a good price. Wrong. We went to six main Contractors for tenders, giving them a full set of plans, a specificaon for Building Services and a specification of all materials from a Quantty Surveyor. The cheapest tender was £2,000 per sq m. Most expensive was £3,750 per sq m. Options you have are: turnkey (full prefab house from Germany/Sweden for example); part turnkey (where they construct and handowver at 1st fix or 2nd fix eg Ireland/Melgium); main contracor (see above); use your own project manager who brings in their subcontractors; or manage/build it yourself. Again, I would choose my preferred construction option before selecting an architect. (In fact we have gone with a project manager and it is coming out below £1,500 per sq m and that's to a pretty good quality).
4. We ended up with an Atchitectural Technician, who I paid by the hour. I didn't detect in lack of skills on my AT's part. I certainly wasn't prepared to pay on a percentage basis and I think if you try and go fixed price its all liable to get hairy as you start changing things from the original design (and change things you will!!). The relationship here is critical, obviously. Keep looking for one until you are 100%. Make sure the contract allows you to terminate at any time and retain IP over the plans to date. Final tip: make sure they have the capability to render the plans in 3D - when you can see and navigate through the virtual building from a tablet it tells you so much more!
5. Set aside many hours to read through this site (and refer to its forerunner ebuild.co.uk). I would not have been able to build my house to the quality desired without it ! Forum members are incredibly generous with their time and advice.
6. If you can, try and identify someone (from this site?) who has done this thing before (or is in the middle of doing it). If at all possible, try and visit them to get some feeling for the big picture. Again from experience, we thought the architect would bring all this to the table, but in point of fact, they don't. You need to hone in on some of the big decisions, and only then fire off the Architect to go through Planning again.
Very best wishes - everyone should try and build their house once in their lifetime :-)