I'm looking for a new camera to buy. Currently, I own a Nikond3400 that I use a Sigma 17-50mm 2.8 on and the quality of the camera itself is no longer cutting it. I was looking into a Canon 6D mark ii at first but have moved more towards the Sony A7iii. The only thing holding me back now is the fear of buying lenses (yes I know sounds stupid) I have not really delve into lenses before except when I bought the Sigma, but I'm just trying to get a lens that will give me a crisp look to my pictures. (not to worried about AF or should I be?)

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My question is if I go through with buying the A7iii would it be smart to buy it with the kit lens or something else? I'm looking for a lens for both portraits and landscapes so I don't have to put a lot of money down. or would it be worth it just buying an adapter for the Sigma lens?