Alex
As the Boardman has an integrated headset that defo has to change, I would say you’ll be fine with a Hope, but as per Mike I’d spend a bit more and go for a CK if you can afford it. Mine’s on it’s fifth or sixth year now still running fine, shop around and they don’t cost a lot more.
Next, seatpost, looks like the Boardman is 27.2mm so you’ll be fine swapping that over (Double check diameter though), and the Soul comes with collar so you’re fine there.
As others have mentioned, front mech. I run Shimano so have had no problems with changing diameters as it’s one fits all now with a couple of shims, not so sure on the SRAM stuff.
Other than that you might as well go for new inner and outer gear cables anyway, and if you are really unlucky you won’t get away with the existing lengths of disc brakes hose – unlikely though.
With regards to the wheels, if it was me I’d be looking at a set of new Hope Hoops, Pro II hubs with Stans Flow rims. The Hope front hub can be set-up for 9mm QR, 15mm or 20mm. I had the Pro II hubs before on Mavic 719 rims, really good, in fact they were about 5 years old and have moved on to another home now, still going strong. I was going to replace with Hope Hoops with Flows, up until the point I decided to treat myself to CK hubs laced to Flow rims instead 😈 Only criticism I have of the Hopes is the front bearings didn’t seem to last too well, but the hubs are very easy to service yourself.
Forks are a little more tricky. Personally I am a Rockshox fan, but still had a good chat with a few people. Very nearly went Fox, but in the end went with Maxle Rebas at 120mm. Having had the Maxle on my Pikes, I love them and defo wanted to go down that route. From experience I’m happy with the servicing intervals on Rockshox, but then I’ve heard plenty of good stories about Fox. Dobby went with the Floats on his Soul and having ridden both, there’s not a lot in it. Did like the Terralogic he had on them though 😉
Personally I found adjustable forks a waste, the only advantage was being able to initially run the Pikes on a frame designed for shorter travel. Once they were on a 140mm adjusted bike, that’s where they stayed. If you’ve paid for it use it. I think there were 2 times I recall winding them down for really steep climbs. Bit like lock out, I don’t use it, I’ve paid for the suspension so I’ll use it. My Rebas came with a poploc and I haven’t fitted it. Anyway, veering off a bit there. I tried a Genesis a while back with 120mm Maxle Rebas and loved them, very stiff and not hugely different to the Pikes, it made me question if I really needed 140mm. So when I went with the Soul I decided to go with 120mm and haven’t missed the extra 20mm so far, seems fine for everything I normally ride. I think 120mm suits the Soul perfectly, oh and have no clearance issues either.