Lights - It depends on a couple of things. a) are you already getting enough light to grow what you want? and b) how long do you you want to extend your growing season? If you want to grow in late fall or in the winter you'll probably need some supplemental lighting. If you want to get an early start on the season in the spring, you'll probably want a light rack for your seedlings.
Water - Having water in the greenhouse is really nice. It saves you from hauling water in from elsewhere and it allows for an automated drip system. I would put a water outlet in the greenhouse if I could.
1. Gaps - Maybe something like this:
http://www.glassstructures.com... (not a recommendations, just an idea)
2. Yes, you could use drainage pans. Depending on the slope of your greenhouse (if any) you might need to figure out a way to keep it drained. You could end up with standing water. You could also put in a sump and a sump pump. Those can also be messy and can be prone to mechanical problems. Or you might be able to put in a drain. That's what I'd do if it was possible. But I'd consult a reputable plumber on this issue.
3. With a greenhouse its never too late. You can plant greens, herbs and other cold weather crops. Google "cold weather crops" for ideas.
4. Yes, open your vents and keep an eye on the temperature. You might want to install shade cloth and/or a fan or two. FYI, people without a greenhouse think the challenge is keeping it warm, in reality the hard part is getting rid of the excess heat during the warmer months.
5. Shelves and tables are great as they expand your space and keep things neat. Also think about hanging shelves and pots from the ceiling. Looking at your pictures, I think you could definitely put in some shelves against the lower half of the glass wall.
(Not my greenhouse)
