I personally would not have a problem using a three point seat as an infant seat. As long as the straps are at or below the baby's shoulders and hold the baby down in the seat, my feeling is it's perfectly safe. From what I understand, 5-pt infant seats in Europe are very new and few and far between, and 3-pts are way more common.
I think I'd go with that Rock a Tot, I know a few people who've had them and loved them :)
Then when you come back here, you can look for a good convertible or a bigger five-pt infant seat, like the Companion or baby trend adjustable back seat.
Julie CPS Tech and mom to 2 in seats