So, big bike frame, with good wheels (17 stone is 238 lbs.)
When looking at used bikes, look at the quality of the brakes and gears, you can always upgrade them of course, but if the bike started out with reasonable quality components, it's an indication that it was a fair bike when sold. The better quality the bike is/was, the less adjusting/maintaining it will need. You'll still need to work on it, or pay someone else to work on it, but minor adjustments will be less.
Good quality is something you will learn to recognize with more exposure to the stuff, shiny does not mean good. Look for solid, well shielded bearings and pivots, brake components that bend in the breeze, are reserved for the very low end, or extreme lightweight racers, neither of which you should be interested in. Consider that you'll probably want some means of mounting full mudguards and rack(s).
Get one that fits you, not easy at you height/size. Any good boot sales around? ( flea markets ) A big frame older Raleigh would be nice, but you'll lose out out on modern shifters and the like. Stay away from suspension, you don't need it, it adds weight and complexity for next to no gain for what you need. Better to get a road frame that can take 35mm plus tires, and run them at about 60 PSI.
If going older, stay away from steel or chrome rims, they suck, too heavy, and braking is crap.