Thinking about a new greenhouse design - what do you think?

[yummersetter]

I want a replacement to my 35-year-old greenhouse before it collapses completely and I want the new one to last as long ( thinking, it'll probably see me out). My OH is a master shed builder so it'll be his next project. We have a friendly double-glazing/ conservatory company in the village so I'd like to involve them and also use some of their discards.
The space will allow it to be 12x8ft or so, long side facing North South with a path on the East side, fence on the west.
I'm thinking of having a 24inch high block wall on 3 sides with a wide double door to the east. Then I want a 5ft high row of double glazed panels on the south side, in PVC conservatory fittings including opening windows/vents for the bottom 12 inches, so an upside down 'normal' window. The north side d/g windows will be 6ft high including 12" windows at the top. The roof will slope down with a 12inch fall from the 8ft North side to 7ft South, maybe double glazed glass or PVC. The west side could be solid wood. There's a high hedge 40ft to the North, a cattle shed to the west and a polytunnel east so the prevailing winds will come from the south.
I've been looking for a computer programme to draw this up but haven't been happy with Sketchup, does anyone have suggestions? And about drawbacks to the entire concept?

[Capsid]

Can't help with the software, but the plan sounds like a good one to me. The expense might be the drawback but if you can get cheap discards then that would help. I presume you want to heat it in winter, hence your desire to go double glazing? If not then I don't see a benefit. Ventilation is critical but I see you have that well covered too. I'd like to see the outcome and how it works out.

[veggiechicken]

Would the west side be better glazed as well? I know you said there was a fence alongside it but I can't help feeling that just a bit of light from the west might even up the growth. Also perhaps a window on the west side would give you a through breeze when the doors are open?

[yummersetter]

Just too little light to be worth the expense of glazing the west side, as there's a six ft fence behind with a fifteen foot high (black painted) cowshed ten feet behind that. There could easily be a vent in it though, that's a good suggestion. Its quite nice to have a solid surface to attach wires and hooks to. I'd like to just keep it frost free which I hope would be less expensive with double glazing. It's not a cold spot anyway.