Now, about the type of snorkel: Avoid,
like the plague, any snorkel that says 'dry'. This will almost always have some kind of flapper valve in the airstream. I remember having one as a kid that had a ping-pong ball for a valve. You REALLY don't want anything in the air stream. Second: if you have to accept 'semi-dry' like the AquaLung Impulse 2 or 3, it's PROBABLY ok. You might not be able to find a snorkel without some kind of useless doo-dad on the top of the tube.
You absolutely want the purge valve and I would always prefer a flex version (my Tekna's don't have that feature). It makes it easier to get in and out of the mouth.
In our herd, there are:
Aqua Lung Impulse 2 Snorkel (Flex version) and
Aqualung Impulse 3
plus my Tekna.
Richard