If I were you I would not spend a lot of money on a pfd or paddle at this point. I don't mean go out an buy cheap ones, because generally more expensive paddles and pfd's are just nicer to use and wear. But, your heights (torso and arm), boat width and boat depth all are important considerations on paddle length, and the height of the seat back in most recreational tandem kayaks will have some impact the comfort of many pfds.
So, if you are looking for a used kayak and don't specifically know the model you are going to find, I would wait on paddles and pfds. There are a lot of used paddles and pfds for sale at reasonable prices too. Maybe go to a local shop and rent some different pfds and paddles to see what you like and what will work well with your first kayak. Talk to the staff. A good shop will have at least tried or used the products they carry and should be able to provide good feedback on various models. Personally, I would avoid the big box stores. Also, some outdoor specialty store chains may give you good advice, but it's really a gamble finding a sales person with any experience.
If you find you really like kayaking and do upgrade boats you may find you're ready to upgrade paddles and pfds as well. I went through four different paddle lengths over a few years before I found the proper paddle. All of these were good quality paddles and I still have several of these I use as demos and loaners for others to try. Same thing with pfds. I'm on my third pfd at this point, my first is used in the pool and has been badly degraded by chlorine over the years, my second is set up for canoeing and my third for sea kayaking.
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