My wife bought a shiplap Tiger Shed
https://www.tigersheds.com/ for her allotment (8 x 6, pent roof IIRC) - apart from the bright orange colour (it's now been painted back with Cuprinol Garden Shades Ash Black) it's pretty good quality - but, being a bit of a woodworker:
1 We laid paving slabs level for the base
2 I made a "plinth" from 3 x 2 treated timber to mount the floor onto, getting it off the ground.
3 The whole thing is DIY assembly and they supply a bag of nails - I drilled and screwed it together . . . partly for strength and partly if she gave up the allotment we could take it apart easily.
4 The roofing felt supplied wasn't brilliant - I laid that in strips the short way (tacked to the roof with alloy nails) and then with better quality felt fitted another layer, lapped, on the long direction. The edges of the second layer were turned over the roof edge and fixed with treated battens screwed on.
Subsequently I've made a plywood panel that fits over the windows (inside) with a rail at the bottom and some wooden turnbuckles at the top - partly to stop the light coming in and also prevent people seeing what's inside.
For all that - I wouldn't very happy to store bikes of any value in a wooden shed as, even with insulation, it's going to be damp and of course vulnerable to theft.