I have 4 small greenhouses joined together, and a separate shed.
The reason I have 4 is I got them cheap when a diy place closed down, but they have served well as I can keep them at dirrerent temperatures, one with doors almost closed for maximum humidity for growing cucumbers, and doors open for tomatoes etc.
One serves as a solar oven for kiln drying wood, it currently gets to 130f in there by day, plenty of ventilation helps the wood dry quite quickly.
I find a separate shed useful. If the greenhouses were attached to the shed, the light would tend to come from one side and the plants would tend to leana bit, just a thought on that.
One greenhouse I use for propagating, and have a home made electric hot bed made of soil heating cable in damp sand in an insulated wooden box. This is great for starting seedlings off, and has saved a fortune on buying stuff from nurseries. I raise so many tomato plants I sell the excess from a barrow every spring......
As to glass vs polycarbonate, glass is very long lasting and tends to be cheaper. Polycarbonate is vital if you have family who like ball games in the garden. Nothing more irritating than enjoying a barbie and someone kicks a ball through your greenhouse.
DIT greenhouses tend to look diy, if you want an aluminium one, it is probably cheaper to buy off the shelf than but the extrusions and cut them all to size, kits will include the glazing bars and door kits, all fiddly to make yourself. Unless you are a much better diy guy than me