Test your Cat5 (if you have some) over 60m, a big loop round the room will do. Then if you're happy with the setup stick it in conduit buried at the appropriate depth.
As it's a one off type job I'd put two lengths in the conduit or at least have it that I can pull another through sometime...
Another vote for Nilfisk. I have an old Wap Aero 820 orsomethingorother and can't fault it. Bags are hard to find now. Last lot came from Amazon Germany.
I have a Bosch GCM 10 S which is very nice but it always tripped the MCB until I fitted a C type MCB.
The saw is about 10 years old though so the newer ones may be less fussy.
For a budget DMM you might want to try one of the UNI-T models.
I have a UT61D which, although not as nice as my Fluke 111 works just as well for most measurements and is reasonably well made.
For reviews of budget DMMs see the EEVblog.
Aside from the construction detail of the shelf there might be a couple of points to consider re dangling bikes off of things; the front wheel will tend to flop left or right and will need some means of retaining it in the straight ahead position if the bike is to sit vertically and the front...
From memory, and this is going back a few years, I seem to recall that the adult wood worm tunnel out of the wood and go walk/fly about sometime around about May. At which point they alight upon a tasty piece of untreated timber and burrow in for a giant feed on your best bit of juicy...
Had a look at your planning application online and so far it all looks very much like the Approved Elevations. Only thing I can think of is your Velux windows (as drawn) might not fit your truss spacing.