My approach tends to be "write what you know" - though I do have rather broad and esoteric interests. This can be a bit of a problem when you get overly distracted reading interesting related articles. As such, I thin your idea of limiting the search by time and number of items is a brilliant one.
Speaking of limited time - is it a good idea for new writers to start with a challenge? The whole point of a 24 hour game is to prove making RPGs is potentially quick, and cutting through development hell that can often plague long-term projects. Stop worrying about making the perfect game that improves on all the others you've played, and even if it fails - you've learned something. I'm really not sure about this straight to the deep-end approach, and hopefully with your guide uses can use whatever time limit they feel necessary.
Something like game chef, or re-using a prior 1km1kt contest might also help limit the scope. I don't want to discourage grand ideas - my first game was a hard sf space opera - but that game is also unfinished because it got too big. You might need to remind new designers to start small, or that many professional games are written by a team.
An aside: mention somewhere in the guide ideas for working with others on the game.
Part of me wants to provide the first-timers kit. Some sample mechanic styles and an explanation (ie 1d10 vs 2d6 vs poker deck), a list of attribute/stat names or ideas for unusual ones (temporal presence, shatter, BAC) and a week long schedule for writing your first game. But that would probably produce way too many similar products to really be useful.
I don't recall if I've show you this link before, but the might be useful.