What zone are you in?
In the cooler/colder parts of the country, growing warm-season veggies like squash and tomatoes in wintertime requires spending a lot of money on extra construction details (insulated panes, etc) PLUS a buncha money on heating. IMO it is not really worth it when you could be growing cool-season veggies like lettuce and spinach and peas for a
lot less outlay
The main issues with any greenhouse are its tendency to overheat during the day and, er, overcool

P) at night. Maximizing thermal mass inside the house helps some, but not if you want, like, tomatoes.
At a minimum, you will need flooring (at least a thick bed of gravel), the greenhouse itself of course, and unless you have nothing to do with your time except sit around and tend to your greenhouse you will also pretty much need thermostatically-operated vent openers and/or fans.
In all but the most southern climates, if you are really set on having a warm (not cool) greenhouse over the winter, it is strongly worthwhile building it as a lean-to against your house so that both house and greenhouse can benefit from each others' heat holding capacity and insulation factor.
Good luck,
Pat