Wanted metal shed....

Argos & Woolworths & tesco have a few have you searched here ?

Can't offer any advice though !

The slabs will be fine, just a note of caution in regards to metal sheds though as they tend to "sweat" ie. condensation forms on the walls/roof very easily and then drips onto anything inside, this can cause a build up of mold/mildew we've had the same problem with a brick/concrete one whereas it builds condensation on the ceiling then drips down I have had to replace my punchbag and some material backed garden chairs because of this, a solution ive found effective on mine is cover the ceiling with 2 inch thisck polystyrene insulating boards thus stopping the ceiling being cold enough to attract condensation formation.

Ideally in your situation I would firstly lift the slabs and place some waterproof membrane down before re-laying them, or lay a decent and substantial concrete foundation (9-12" depth minimum) with plenty of overhang all round as the problem of sweating would be caused by the shed heating up and drawing water from the ground which will form as condensation in the evening as the surfaces cool.

Hope this helps.