However you go, it's fun.
I built my first greenhouse using wood from old snowmachine pallets and roofed with the ripple stuff and visqueen walls. The windows and vents were scavenged off trailers. I got two big galvanized stock troughs for heatsinks and painted them black on one side and put tops on them for benches.
Even in a viciously cold area (Pinedale, WY) it was good from mid-March- early November, when I'd shut it down and drain the heatsinks.
Just put the main heatsink in the new one:
It's a 400 gallon stock tank, with foamboard bottom and top, and thermal bubble wrap doubled around the sides. The PEX tubing under the main floor will loop through the water in the heatsink. There'll also be a loop to a solar collector, for winter.
There'll also be four 75-gal black plastic tanks on the rear wall just above this: lots of heat storage, to take advantage of our sunny winter days.