I don't think there is any one formula that fits all. I never rented a kayak in my life, but I now have four of them. When I first became interested in kayaking, I never in my most radical dreams would have thought I would have the boats I have now, nor the paddles and other gear.
Most people probably start out with a beginner boat of some type and either progress from there, or maybe just stick with what they start with. I have friends who hardly ever paddled a kayak, but they caught the bug and started out with a brand new full blown composite sea kayak. The latter is probably somewhat rare, but knowing what I know now, that would be my plan if I were just getting started. That of course puts the cart before the horse for most of us, because there are precious few ways to "know what you know now."
Renting, or having a friend with a loaner probably works best, but there are demo days put on by dealers where you can get a decent taste for this wonderful sport also.
My best advice is to visit several real kayak stores--as opposed to the general sporting goods stores that just happen to carry a few kayaks. Browse around and get familiar with what the real deal is. Don't let the prices and enormity of choices overwhelm you. Most dealers will have folks on staff who are enthusiastic paddlers who will answer your questions and should be very helpful when and if the time comes to pull the trigger.