Better to be safe than sorry especially if your site starts generating some real traffic and income from the ads.
Also, I think your idea has a lot of merit and could be developed into a community, lead by you, that assists users with installs of PHP scripts and open source projects.
Maybe create a system where someone can sign up (members), assign scripts they are profecient in installing and people viewing the site can do searches for individuals that could, say, install SMF forum. Also, allow user to rate members this way the bad will be quickly weeded out. You can even provide a first level of approval based on something submitted to verify a person.
You would only need a small group of individuals looking for increased exposure to make a very functional site.
Who knows, maybe DonationCoder would stand behind you (its all about networking and setting up relationships).
Anyway, enough rambling for me.