Question: As I prepare for fall around here (USDA zone 8a, max low ~10f, average low ~45f) I long for a greenhouse to bet overwinter some of our showy pots, and perhaps get a jump on some annuals and veggies as next spring rolls around. It's no small task to add a greenhouse to our property (plus I have to check local building codes to see if it's possible to locate it where I want it. I'm happy though because it gives me lots of time to plan and budget. I foresee something with a rigid frame, about 8 ft x 12 ft. I have power and water close by.

I'm wondering what systems people would suggest I install? Are there some basic ones I should have (say a heater, and auto ventilation)? Maybe some add ons I should consider? Stuff I probably should avoid?

Answer: 
Easy as can be. Seeds are not some magical movie monster that cannot be killed.

Put them in a pan, bake at 250°F / 120°C (or higher, but that should be adequate) for an hour, they should be good and dead.

Or - wrap in aluminum foil, place in fire, cover with coals (if you don't want them in your oven/house) You could also do the foil wrap and then put them in your oven if merely containing them will keep you happy.
Oh shoot. If you are looking at dried seeds it is way too late to do anything at all about deactivating those seed. Sorrryyy. The best time to stop weeds is before the plant is able to flower or shortly after the seeds germinate. Baby plants easily killed.

This plant is a biennial...yes? So easy to eradicate by 'weed wacking' using a line trimmer on less than the highest rotation...talk about that later. Or using a hula hoe. Or using a chunk of tarp and scooting around on the surface smooshing baby weeds. Get the chunk of tarp fitted with slots so you can secure the tarp to your feet just enough to hold. Change foot slots and scoot around some more.

Trust me, I'd do this at night where no one can watch!

If there are areas such as gravel driveways or paths, you can use garden strength vinegar, I think it is called 'weed block'...changes the pH to ACID where no plants can germinate much less grow. Nothing grows. To make the soil usable again you lime it and bring the pH back up.