2020 ideas

[Socksnsandals]

If you're only on a £100 budget then a (new - SH may be available "buyer collects" as they are a pain to take down especially if oxidised / rusted together) GH will be out of budget which also means that the need for a base is reduced. Because the aluminium GH are pretty flexible and made to relatively tight tolerances they need to have a level base or the glazing spaces will be rhomboidal rather than rectangular and the glazing material won't fit.  Also, though your garden looks fairly sheltered, polycarbonate is more likely than glass to blow out of the frame in strong winds, which a less-than-square base GH would exacerbate - at least with polycarbonate you wouldn't have a load of sharp shards to collect though a friend's glass GH in an exposed spot blew away and shattered a sheet over a neighbour's car.  However, if you are going to make one like BG's with the polycarbonate sheets nailed to an intrinsically flexible wood frame you should have no trouble from gales as the PC panels will be securely fixed (use the big galvanised shed felt roof nails or longer smaller headed screws with washers).  The lower edge of the frame in contact with the soil will rot with time, but all of it will be prone to rot as the inside will be wet with condensation on cool days / nights and at that price you can afford to rebuild to an experience-improved spec in a couple of years.  In my original post I commented about access to the shed being a pain if you were to position the GH in the centre of the garden - I meant if you were going to have to go through the GH to get there but you might have space to take e.g. a wheelbarrow down either side if you positioned it centrally in which case that wouldn't be necessary.  

Smell might not be such a problem with a plant or two but your drawings are for a fairly substantial GH which could accommodate a stenchtastic plantation...