Wot Robin said.
My grandfather used to sell a lot of genuine "Larch Lap" fencing panels, and held the view that's all it was good for (but it is good for that purpose). It is pretty resilient, but picks up algae fairly easily, and nowadays you can't officially re-creosote it yourself (at least until we've Brexited!). And, yes, it will split if it notices you staring too hard at it.
There's two reasons your local mill has it cheap. One is that it's what they process (presumably for fencing panels), and the other is that there's a lot of wastage they want to shift - knotty, or with shakes or useless, gnarly grain, etc.
I'd have a good rummage through their offcuts, to see what the overall rejection rate is likely to be.
Be pessimistic, and work out how much you'll have to over-order, to allow for both your own off-cuts and unusable stuff. Also, if you buy it treated (you might not be able to), what are you intending to do with the waste? It tends to spit quite a lot on the fire.
Might have missed this: are you intending shingles or horizontally running lapped boards? Well done shingles don't trap water as much, so probably don't rot as easily and let you get more out of knotty boards, but they are much more effort to fix (but easier for one man on his own). And, of course you need more overlap, so more material.
Drill, and use hot-dipped galvanized clouts (gently!), so you get the most durable fixings. The more it's able to move, the less trouble you'll have with splitting.
There are waney-edged clad buildings over in the Surrey-Sussex area that have lasted for decades, I think most of them will be pine or Douglas fir from local plantations (or possibly elm) as there's a lot of private and Forestry Commission land over there, but they were thoroughly pickled in creosote, or a tar/creosote/oil mixture, back in the days when you could (a) use creosote, and (b) had chaps around the place who could do the job annually, and (c) the trees were bigger (so the boards were better), and (d) they were outbuildings, not habitation, so nobody minded the smell (mostly).
Personally I love the smell of creosote, but it's an acquired, er, thing. Anyway, accept modern substitutes and be prepared for disappointment.
E.
