First thing you should do is consider your price range. I bought my greenhouse from the greenhousepeople as well. I have small children (5 and under) so it was poly carbonate or safety glass. I went for a 12'x10' GH, so although it costs a bit more, the safety glass was what I went for. Quite simply at the size you are looking at, if you get stronger winds, plastic will likely blow away. The weight of the safety glass is a real bonus, I shudder to think how much I would have needed to replace with horticultural glass even.
I actually put mine on paving slabs, which not being a builder were a real pain in the bum to lay level. With hindsight, I'd have definately dug a trench a foot wide and about 8-9" deep, put some hardcore in, then used some of that self leveling concrete mix, loads faster and just as good. You need it to be level otherwise your glass won't fit properly. Once you have the frame constructed, check the dimensions across the diagonals, also run your eye along the sides to make sure they are straight and not bowing out (which is the typical thing).
Make sure when glazing, the day is not too windy, and that you will be able to fully glaze in a single go, even a single pane left out, if the wind picks up can destroy a greenhouse in a matter of minutes! (Well cost you a packet for more glass!)
I myself went for the bar capping option as opposed to clips, it gives nice clean lines, and of course holds the glass tightly across the entire length of the glass, which gives added stability.
Next up, consider ventilation, are you at home all day or out for long periods? If you work like me, then automated ventilation is a must. With the GH size you are talking about, one louvre window is a must, personally I went for two, one for each side which sets a nice cross breeze through. I also have them with the automatic openers. My green house has 4 top windows, 2 of which are auto controlled. You will find these things are quite expensive probably 200 quid or so, but if you are out a lot, worth every penny for peace of mind. Should last years with 5 mins of maintenance every now and again.
Regards to staging, greenhouse staging appears to be massively over priced, if I had the time again I'd be looking at local ebay or gumtree. In the meantime, consider those 4 tier green houses, currently in homebase for £22. The added bonus is if you pop the covers on in winter then cover with bubble wrap, should make nice frost proof shelters for overwintering things. I have to be honest, my staging only stays in the gh over winter til start of summer, they only ever hold seed trays so as for being strong, the 4 tier will be fine. In the summer all the staging comes out to increase the growing room for toms etc.
Anyways, good luck with your GH, I hope I have covered the main bits, any questions you have I'll check back on here for next few days.