As in everything else in life, you get what you pay for...
...I have used the corrugated sheet to roof the pergola over our small piece of decking (Wicks is the cheapest if you don't have a local hardware store that knocks it out at silly-cheap prices like the place in Norwich when I built my brothers') and it is perfectly ok so long as you build in sufficient support; and make sure you use the proper plastic roofing screw caps...http://www.ebay.co.uk/itm/100-PACK-CORR ... 0615256636
...or the sheets will crack (drill holes in the sheet first with a new, sharp bit) and the first time a strong wind blows will be the last you see of your nice new roof!
The only downside to it is the crap that still
manages to creep under the overlaps between the sheets and resists all
efforts to clean it out!
If your Missus can live with that without constantly nagging you to clean it out then all well and good; if not I would recommend spending a bit more and using the box-section stuff that butts up edge to edge and sealing it with good quality
clear silicon sealant.
As for the run off (you are not allowed to build/install anything
that encroaches on a neighbouring property in any
way) you just need to run some plastic guttering along the edge into a downpipe and into a waterbutt. You'll be amazed how quickly it fills up!
That way you will also keep your water bill down when watering the garden
If you are building on concrete/Tarmac hard standing I would use bolt-down Metposts to stand the outer supports in; and find somewhere that sells cheap 3x3 fence posts for the uprights as you'll save a small fortune and they're ready-treated.
Apart from that MikeD's photo above is the ideal 'blueprint...'