Scores are glitchy
I played through the game without purchasing anything beyond the base equipment to see if it was possible, and it is, except I revealed a bug. I failed missions 6 and 7 a few times because for some reason shooting the guy in the face wasn't counting as a kill, and then I finally succeeded, and all seemed well. The problem is every level after 7 gave me the scores from level 7 instead of the scores from the level I'd just played.
Also the scores don't reset after you finish playing once, so if you want an accurate score the second time around you have to close the window and re-open it.
Those two issues not withstanding, it's a very good game and I look forward to playing the third installment if there is one.