One long side of it would be fairly close to a 5' stone wall (a couple of feet away) but that shouldn't really affect the light too much since the other three sides would be wide open to the light.
If the wall is on the South side I think it would make a lot of difference ... but if on the North it will make no difference.
Later on this year I intend to go shopping for a decent greenhouse
If its "not immediately" I would recommend setting up a search on eBay (i.e. so that you get an EMail when a suitable match comes up). I've had two, large, greenhouses off eBay that saved me a lot of money.
Any opinions on whether it's best to go for safety glass, horticultural glass or polycarbonate?
My view is that you only need safety glass if there are kids about who could career into it (bottom of a hill that they use for their go-karts, for example [
])
Personally I favour glass over polycarbonate as I think the weight helps keep the thing on the ground.
How about vents, double or single doors?
My opinion:
You want convection cooling, not a draught. A draught causes the plants to transpire more, so they have to drink more water and that stresses them more.
To achieve this I think you need the largest greenhouse you can afford / have space for (much harder to keep a small greenhouse cool). Plenty of vents at the top and a few louvre vents at the bottom enable convective cooling (whereas throwing the doors open means that more of a draught-effect is created)
Double-doors help if you want to get a wheel barrow into the greenhouse, if you are going to grow in the soil (rather than in pots / grow bags) then you will have to change the soil, and getting a barrow in will be important. If not growing in the soil then you could get away without a barrow / wide door.
Personally I think growing Tomatoes and other crops in the greenhouse is easier done in the soil. I've done pots / growbags etc. in the past, more time & effort spent watering, and the pots themselves reduce the height-to-roof so you get probably one less truss of fruit. (I have lowered the beds/floor in my greenhouse to maximise roof-height)
Consider if you want automatic openers for the roof vents. I haven't bothered, because I like to open them on the side away from the wind (in mid summer I have to open everything of course), but if you will be away from home etc. then the vents have to be opened by a neighbour / automatic device [
]
Also consider a water butt. Most easily arranged at the back (opposite door) so that the gutters from both sides can be joined to a single water butt (or have two at the front ..). Slope the base very slightly so that the gutters run towards the water butt end
Electricity to the greenhouse? (Heating / light / propagators)
Heating? Gas / Parrafin heater to keep the frost off in the spring? or do you want to heat it in the winter too?
Staging (tables)? Plant raising in the spring, and a collection of Orchids or somesuch [
] - or are you going to grow Tomatoes / Cucumbers / Melons?
Cropping wires? You can get small bolts with funny shaped heads that "slot" into the aluminium frames and have a wire ring on the end - through which you can put a decent strength wire end-to-end in the house, and use that to train Tomatoes up to etc.
Some pictures of my greenhouses on my blog