dreaming in 6 years green house

i can claim my pension £12,000 in 6 years i would like to build a brick Victorian green house against my west facing brick wall. I can probably do it mostly myself and oh. will I get the benefit of it, what are the pros and cons of a green house apart from starting off veg, peppers and earlier toms.

it will be about 20ft wide and 6 ft deep.

i guess i could sit in there too on cool sunny days.

It doesn't need to be a greenhouse as such - a conservatory should give you the same results but with more comfort!