If you can build a bike you can easily build a shed!
Discovery Channel or such had a whole series dedicated to this.
You will get much more than you would pay for doing it yourself.
I would use as much 'glass' as you can get away with, even consider up to fifty percent of the roof in poly. Nothing beats natural light.
Seeing as you are going with a concrete plinth I would say you could safely build in stages, stage two being the exterior.
We did this recently after an old shed collapsed, needing something up fast for garaging atvs, etc.
We bolted post holders into the concrete and had a frame up in a day, skinned and sealed by half way through the second.
Intention to insulate and fit out fully before the winter but it is pretty tight already and things haven't got that bad weather wise this year.
It really is piss easy to build a shed, and you can get everything delivered, usually free, by a wood yard.
Then again, you can get a firm to come in and do it, if you post your location I am sure you might get some recommendations.
