Is it bad if I can't update regularly?
School should always come first, so I would encourage you not to worry about how often you can update. Some people (like myself) do like to create a buffer to work around drawing slumps, but I wouldn't stress too much about it when you're just getting started unless you think it'll help you keep updating, because for some people (again like me) it keeps them more motivated to keep working because the stress of having to hit a deadline on a regular basis can wear them out, while other people, that stress is a motivator, so they don't bother with buffer in the first place.
Regular updates are all well and good, but as long as your readers are aware of what's going on, they're usually okay with whatever you need to do. Regular updates are really only important if you're trying to be a professional (and even professionals need to break up their schedule sometimes)
How much do I have to script or plan before I start drawing and posting?
As @thomassayle said, planning is different for everybody. Some people plan as they go, others plan ahead of time, just as long as you do start eventually, there's really no rules on when you should start. Some of my projects I knew exactly what I wanted going into it, while others, I'm flying on the edge of my seat with the planning. What you've mentioned just there is absolutely enough to start, in all honesty. There are definitely people with successful comics out there who've started with less.
And for that much work, I personally wouldn't try scripting that much before starting. You're talking years worth of comic right there, and I can guarantee you're going to want to change things along the way as you're working on the art that could force you to either want to rewrite it all long before you hit the end, or make you feel like you have to stick with an idea in the story that you're no longer feeling. I mean, if you really want to write that much ahead of time, more power to you, but my recommendation is maybe write one or two chapters ahead, and maybe just keep your rough outline on hand. (But again, some people don't do this. One of my comics, I don't even have a script for it, because I've never felt like I really needed one. I just thumbnail the chapter and my edits come in the form of taking my thumbnails and placing them in my page files and looking over the story again, and then a third time when I'm cleaning the roughs up. Not my recommended way of doing things, as scripts are helpful for helping you feel like your story isn't going off the rails, but its doable)
Is that too ambitious for my first comic?
My honest answer is yes. If you want to make sure you're going to finish it you'll want to start smaller, BUT!!!!! I also think if it's what you want to do that you should absolutely do it. Being really ambitious helps you learn what your limits are, and dude, some people their first comic really does end up being a thousand page epic. Who says you won't be one of them? I would never have learned anything about my own limits if I didn't push myself to the extreme, so I am not one to discourage others from doing the same.
If I intend to print my comic eventually, should I think about that when I'm deciding on canvas size?
Okay, this one you do need to keep in mind ahead of time. Standard comic pages are done at 11x17, and you'll want your DPI at 300 DPI or higher (I work in 300, some work as high as 600-700). If you're planning on doing your comic in color, some would also recommend you stick to CMYK and just change your jpgs or pngs to RGB when uploading (CMYK doesn't always work when uploading on the web, but what you send to printers you'll want to leave in CMYK). You CAN stick to RGB color mode as printers are getting to the point where it doesn't really matter which you use, but if you want to be safe, use CMYK (this does change how colors will look in your paint program so if something isn't showing up exactly right or how you want it, this is probably why. CMYK's purpose is to avoid colors that printers can't print)
If you want to be really fancy you can also look into methods of preventing color bleed and all that, but that's something you'll probably be okay if you don't really get into until later. I'm pretty sure most webcomic artists aren't even aware of these and again, they're mostly methods that HAD to be used in the past because comic printers were a lot shittier than they are now.
Do most of you draw your comic on a larger canvas and make it smaller to post here?
Absolutely, even folks who aren't planning on printing. If you want to print your comic later and want good quality prints, this is pretty much a requirement, but if you're just doing whatever and don't care if your comic makes it to print, it doesn't really matter if you do or not, but it does make it easier for if you decide to post your pages on other comic sites or even make your own site, as size requirements and recommendations are different depending on where you are.