Aye, don't go cheap. I bought my 5m x 2m shed 1 month before I started work for a nails company supplying shedbuilders

£800
1) Pent for all the reasons given plus simpler construction leads to less possible points of failure.
2) If just for storage, don't have windows but if you do, make sure they're properly installed (not a sheet of glass with a single bead of glue running mostly along the bottom)
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3) Check the framing. Mine is 2X2" rough sawn white softwood.
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4) Roof - should be boards, not planks. Planks shrink and stretch the felt. I currently have a tarpaulin roof
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5) nails - ask about the galvanisation. Triple Life or Hot Dipped is good, Stainless is best. Bright nails or barely galvanised will leave streaks
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