At 6'5", bike size will be a problem. 23" is the largest common size but you may find 25". Cannondale used to do some good XXL sizes. Finding something big and cheap is impossible, big and used a possibility.
Some high end custom frames have been made for tall guys, again these are rarities. The goto guy for big, custom-bikes is Leonard Zinn but his informative website seems to be malfunctioning.
If you get a decent 23" you may need to add an extended seatpost, the strongest you can find, nothing lightweight, and a longer and/or higher stem, the fattest you can find. You can bodge it to fit you and live with any handling issue till you can afford a $2000 custom frame.
You will need tyre clearance for some fat tyres, it wont make a big difference whether you use 26" MTB or 700c for you riding. I like Schwalbe Big Apples, they are fat but slick and efficient.
Suspension for a heavy rider is more to protect the bike than the rider. If you slam into a pothole or bump, suspension will distribute the impact over time, reducing the peak load, saving your frame from a beating. Cheap suspension is not really up to the task so its better to let fat tyres do the job.
I like disc brakes, they are grippy and work in all conditions. This is not a deal breaker.
I like steel bikes, but again, it is not a dealbreaker for big guys. Big frames are naturally bendier than small ones but you need extra stiffness, all you can get. Aluminium frames are built stiffer than steel which is a Good Thing for you. Mid-market, factory Alu frames are strong and reliable.
Wheels are always the weakest link on a bike so get the strongest you can. 36 spoke with heavy duty aluminium rims on a midrange MTB hub will be enough. Any more spokes is a fancypants specialist item. Any less is less than you need.
If you get a bike that fits a bolt-on luggage rack and fenders you can use the bike for shopping, errands, commuting ass well as fitness. You will ride more, pay less for gas and enjoy your journeys more. The budget then becomes a bit more flexible when you add in the utility.