It is hard to recommend a device for you as everyone is different and wants different things from their PV. There are so many different PVs out there and they each have their own pros and cons, so I'd advise thinking about what you'd most like (i.e.--looks/feels more like a traditional cig, easy to use, long life batteries, the list goes on). Then I'd advise reading through the forums a bit and reading what other people think of their purchases.
That said, for me I started with a 510, but found I didn't like having to deal with things like refilling or trying to modify carts to get them to do what I wanted. So I recently switched to a KR808, which seems to be a good fit for me so far in that I can either choose to refill the cartos or just use and toss. But as I said, your needs might be different than mine.
As for where to buy, my best advise is to look at the suppliers who are listed here as "registered suppliers". They all have been proven to be good, so take a look around and see what they offer and at what price.