If I want to start developing small indie-like games with XNA on Xbox (I don't think any game I'd make would need more than 100, maybe 200 megabytes), will the 4gb Slim (I want to get a slim version because of the wi-fi) version of the console be enough, or should I get the 250 gb version? I don't really plan to play actively on the console as I'm more interested in developing on it. Also, will a console and a verified student XBLIG account be enough to set up everything I need to run and debug my game on the console? Thank you.
The answer is no.
You'll need an actual hard-drive to use the XNA connect application (upload your game) - but not to play other games.
You'll need a physical hard-drive - NOT flash memory to deploy games.
If you have a AppHub Membership (from DreamSpark, you say yours is from); this is all you need. You will link it with your Gamertag and download the Connect App and and you'll be all set from there.