Well Id put it up first before you worry about the glass! if your anything like my husband the lot will need replacing by the time its in place!!
You can easily get replacement glass.
One we put up we made a base of paving stones first.

I did the same I used polycarb because we have a new grandson. Vents are readily available on EBay including auto vents for around £20/25 and they are easy to fit also have a look for a PDF instruction manual that will make building much easier.

When you dismantle the greenhouse mark each pane of glass with a specific number (we stuck a piece of masking tape on the panes and wrote on that). Then draw pictures of the greenhouse on a piece of paper and put the numbers on that. It makes re-assembly much easier.
If the greenhouse is constructed for glass glazing then you will probably find polycarbonate too thick to fit. Check it out.

Depending how much work you are prepared to do, I would cement in a row of concrete blocks on edge around the perimeter and sit & screw the frame on that to give yourself an extra 9in of headroom,otherwise if you are taller than five ft six you will knock your head every time you go in and out