I am looking for info on a greenhouse as a gift for my parents, both passionate gardeners in their 70s. I am hoping that a greenhouse would allow them to pursue their favorite activity year round for as long as possible. In the summer they tend a 20 x 40 square foot vegetable plot, do all their own landscaping and maintenance, and tend numerous perennial beds. They propagate azaleas (species, evergreen, and deciduous -- I don't quite understand all the types) and have collected over 100 varieties which are thickly planted on their 1-acre property.

I would like to construct a fairly permanent, well designed greenhouse, ideally attached to the house so they could enter without going outdoors in the winter. I envision something large enough for them both to work together, where they could grow year-round cool weather vegetables, take care of indoor plants, overwinter tender outdoor plants, plant seeds for summer crops, provide a nursery for small plants -- just a general garden workshop.

Can a greenhouse provide this for them? Am I making any wrong assumptions about how they could use it? For these purposes does it have to be heated? How? What sort of other technical considerations should I be thinking about? Are there companies with semi-stock greenhouses that might be adapted to fit with a house? What sort of budget do we need? Any good books or websites?

This is a lot to ask -- hopefully you can help with some answers and I wasn't able to get much from the forum's FAQ page.