Here is the raft we carried.
They are not cheap. Let me give you some advice on them. This is all to the best of my knowledge (and I have two kids so my memory is not really good anymore), but here goes:
1) Do not buy it until right before you are ready to punch off. If you are just gearing up right now, remember they do not pack (new) until right after purchased. Since you have to repack every 12-24 moths, wait until the last minute. By the way, a repack is better than $500, typically, as I recall.
2) Used raft. I personally do not see the problem with it, if it is not too old. Just have it repacked after you buy (which you will want to do anyway to pack your own stuff in there, read below).
3) When you repack, buy a hand-held watermaker, unless you are 24-48 hours from rescue. If that is the case, it is up to you. Most packers can stick in a watermaker at time of repack.
4) Someone earlier talked about mounting, and I agree. The things are big and bulky and heavy. Be conscious of where you are going to put it and be conscious about how/where it is going to deploy so it does not get too tangled in the rigging.
5) If this is for occassional use, you may consider just renting one??? It is a whole lot cheaper, you do not have to worry about someone stealing it, mounting it, the annual repack, etc. http://www.westpacmarine.com/rentalrafts/default.asp
6) SOme people prefer the non-canister type so they can just throw below on their passages. I can see the merit in this, though we opted for the mounting. Also, make sure the raft comes with its holder, most do not and you will have to order seperate.
7) Sailingdog is usually right about things, but I will mention that my piss-poor memory told me to go one size up. If you have 4 people (don't forget to include pets, unless you plan on eating Fido) go to a 6, not the smallest you can get into. But, you may want to double-check that. I cannot remember for sure. I did oversize. I was less concerned about being in 50 foot breakers and being thrown around than I was a couple hundred miles offshore and hitting a whale or shipping container and sinking in nice weather. Thus, the larger the better for me... but that was just me.
Hope that helps. Take care.