Greenhouse recommendations

eBay is your best bet. We got a lovely reconditioned greenhouse from a lovely guy who refurbished old greenhouses, it came with toughened glass and self opening vents. It looked like new. He also assembled it. I think we paid £425. My OH built the shelves and benches, but he did have some for sale as well.

Polycarbonate is my choice for a greenhouse as it doesn't need shading applied and it is warmer in winter. A self opening window is essential as is a potting bench at the right height.
It is a long time since I bought mine but I doubt a base is a floor. More likely to be a rigid rectangle to attach the sides to. You can floor the greenhouse with old slabs which act as heat reservoirs at night or uncovered to allow planting directly into the soil. I have decking squares for my walkway and fine gravel over a membrane at the sides so that I can use the soil if I wish.
Normally late summer sees sales of gardening equipment as stores discount to get rid of stock but this has been an unusual year with gardening being the go-to hobby during lockdown.
Enjoy your gardening.

Thank you both for your recommendations.