Welcome to BYC
Very tidy looking tractor! How many chickens is this for?
To answer #1 and #3 at once, yes you most certainly do, both things. The simplest route is probably to combine them. Cut two window-type holes on opposite walls at the end of the 'house' where they do NOT roost. (This is the tricky part about making teeny little tractors - it can get extremely difficult to provide sufiicient ventilation without having it be a strong cold draft right *at* the chickens. This tractor is going to be
exceedingly difficult to winterize comfortably - have you thought about building 'winter quarters' for them?). I have a similar-sized tractor (tho with a larger 'house' part) for 2, formerly 3, hens, with about 3 sq ft of ventilation, and I wouldn't want to go much of any less than that at *all*. You may need more, since it is such a small air volume in the house.
So anyhow, cut your window holes. Cover the holes very securely with strongly-attached 1/2" or 1/4" hardwarecloth.
Finally, frame out two small windows using plexiglas or the glass from secondhand pictureframes or something like that, and attach them as covers for the holes. SOme people hinge them, some have them sliding in tracks - it depends on your DIY preferences and on how much wind etc your site gets. Either way, you can have them either entirely open, or open just a crack, or on rainy/windy days you can close the upwind one, and on the first few very cold nights of fall (before moving the chickens to, I hope, other quarters for the winter) you can even shut the vent windows entirely.
One thing that would help reduce the amount of ventilation you need would be to have some sort of droppings board type thing under the roost (but see below) that could have all the poo scraped off when you let the chickens out in the morning. This will remove almost 50% of their daily poo/ammonia/dampness output.
The roost thing, though... I am not sure what your dimensions are there, but I am going to guess that the 'house' is no more than 18" high? You may have room for a VERY LOW roost bar, just to keep their bums up out of the poo, but certainly no more than that. You might put it crosswise at the end farthest from the door, and put less than normal bedding there so that you can clean that part of the floor out every a.m. like you would a droppings board?
Two other bits of food for thought: 1) is there a way for you to get into the pen part of the tractor? You really will need to be able to, sometimes. And 2) the chickens are going to try to use the open pophole door as a ramp (that's a pretty big hop into a pretty small opening, otherwise) but they will slip and scare or hurt themselves on a smooth and unbraced surface as it seems to be now. If you want it to hinge down, it should probably be
braced when open, and have a batten on its inside top edge (which will be the bottom upper edge when it is open) for them to use for traction. Or you might consider some of the alternative ways of doing the door, although you may still need to provide them with some sort of step or ramp as an aid to getting back in. Just a thought, because it's easier to tweak the construction BEFORE the chickens go in
Have fun,
Pat