I don't know exactly, but I know that the process is VERY subjective. When I was at University, I was asked about Birthright, and told the recruiter (not really a friend) that I had already attended a similar program with partial funding from Birthright. I was told not to worry, that they didn't really need to check up on that. Basically, they want to stop people from freeloading on the program. I can ask a few friends whom are active in Hillel if there are any specific selection criteria, but basically they just want to see that you are "geniunely" interested in Israel and Judiasm. Fortunately, this is pretty easy, as the majority of reform jews in the US don't speak a word of Hebrew, and couldn't read or recite even the Sabbath candle blessing without a direct transliteration (Sorry for harping, but that's been my experience).
The tattoo's shouldn't be a problem with the selection process (The recruiters are not allowed to be judgemental), but they may cause a bit of animosity/curiousity in Israel: Tat's are, after all, banned in Judiasm. I wouldn't worry, you've got this in the bag.