I guess the question was mostly theoretical -- then again, it's only theoretical for another month or so.
To answer your questions:
* Ruby on Rails, Apache with Phusion Passenger for deployment. We've been using SQLite for development, but probably MySQL for deployment, and are not picky on the flavor of Linux, but like Debian.
* Tough to predict at this point. The software is fairly niche, but it's a large niche (freelance graphic and web designers).
* The plan right now is to start with one server (likely at Linode) and use S3 for file storage (perhaps also backing up to Mosso). But this is something we'd definitely be open to changing based on the recommendation of an expert.
* We're bootstrapping the app and have the biggest chunk of money set aside for legal fees (incorporation and making sure our TOS passes muster). In my head I was hoping this wouldn't cost more than $1000 as a one-off job, but, this is definitely not a place I want to be cheap. I'd rather spend more now than wake up one morning to a disaster because I tried to cut costs at the get go.
* We don't think it should be too heavy.
Planning to run a small closed beta in the next couple of weeks to get a better idea of things like that, though.