The most common mistake people make is getting a frame size too large. You're a tall person but older and overweight so the excessive reach on a large frame could be a problem if you have any circulatory issues, peripheral neuropathy, and whatnot. Back in the day when you knew bikes a frame your size had 565mm effective top tube length but now they try and sell you a frames with 585mm ETT (young wanna-be racers fit). So you have to pay attention to the top tube length and avoid excessive reach. I'm an older guy and about your height and look for 540 to 570 length ETT (effective top tube) frame sizes. I prefer frames on the small side of the fit range. I'm referring to road bike sizes, btw. Mountain bikes are sized differently but I wouldn't buy one if you intend to ride mostly on pavement. You might easily fit a medium sized or medium-large frame, so pay attention to the ETT measurement instead of the designated frame size. Bike sizing is more nonsensical and arbitrary these days. You can fix a too-small frame with a longer stem, but too large a frame can never fit properly.