Hmm, If only there was someone on here who makes a lot of doors...
AJB Temple wrote:How difficult is it to do, assuming I am reasonably competent on the tools?
It should be very simple for a man of your calibre, especially with decent guidance from the good folk here
Since it's basically been treaded by Mike and the bear I won't touch on it too much, but yes, you really do need a prime grade of oak that are dead-straight grained for joinery work. Even then, it will still crack and move quite a bit.
So there are a few choices, it all really depends on budget and how well you want the door to last. There's plenty of suitable timbers to choose from, but here are a few suggestions:
Joinery softwood/Scots Pine is nice to work with and fairly inexpensive but it is relatively high-maintenance to get a decent lifespan out of the timber, as much as Jacob would've stamped his feet at that statement, the timber you get now
isn't the same stuff they had 150 years ago, I've seen softwood windows rot out completely in under five years.
Sapele is a well-regarded joinery timber, solid, fairly stable when the grain is nice and straight, and not silly money either, lasts a fairly long time and is pretty forgiving when it comes to maintenance.
Iroko is another one that is suited to the job but you really need to select the most straight pieces you can find as it does have a terrible habit of twisting and it is a very oily timber, which is great for work that does need to last a long time but it can be a pain to paint. It's a little dearer than Sapele but not a hell of a lot more.
Accoya practically the be-all-end-all timber, if you can stretch to the price tag of around £2400cu/M now. It doesn't move with seasons, isn't affected by water ingress, is guaranteed for 50 years above ground, and certain paint manufacturers guarantee their paint for 10 years on top of it. The only real downsides aside from the price are that it's incredibly soft and not particularly attractive, it suits a painted finish very nicely but is an absolute nightmare to stain, it is also highly corrosive to steel fasteners to the point that they'll rust within a few weeks so more expensive stainless steel hardware must be used.
So, as you already pretty much say, before you get your teeth into it you really need to decide what kind of design and layout you want for the door/s. I'm a massive fan of "keep it simple" and while you haven't even mentioned it, please do avoid thinking about bi-folding or sliding doors. Seeing as your opening is fairly wide, you can go for a door with a light on the one side or both sides as you said, or if you ever see a need to be taking awkward and large items out of the door in the future then perhaps a french/double door would be preferable, at that opening size a single door shouldn't be far off a standard door size anyway so it won't feel too constricted with opening just the one door to go through most of the time.
You also need to roughly decide what ironmongery and locking gear you wish to use as this can affect the way you need to make the door too, not in a major way but enough to affect allowances and section sizes. I like 3-point locking systems as they pull the door nice and tight to the frame top, middle and bottom, but these require a more modern style of handle with a euro cylinder lock (although there are quite a lot of handle options to suit the style these days) compared to a 5-lever lock.
Frame construction is fairly straightforward, but you tend to want the top of the frame cill to be either flush or slightly higher than the floor level of the room it's going into to avoid a trip hazard. Being either inward or outward opening makes a difference too, generally, you will need an aluminium threshold such as a Macclex to sit atop the cill (or directly on the floor for a cill-less frame) for an inward opening door but on an outward opening door you can manufacture the cill to suit. You then make the door/s to suit the frame.
If you want a bit of help with the sizes of the timber sections and allowances etc... for a double glazed system I can draw what I do up for you.