When undertaking an attic or third floor renovation, consider using a crane for the heavy materials. For about $500 a day you can rent a small crane or lift. It can easily handle heavy materials like lumber and drywall, not to mention hot tubs. They can boom out about 42 feet, enough to get to the top floor of most homes.
Welding inside of a shop or garage can create fumes. But they can easily be removed from the area by using industrial-type fans, or you can make your own from a recycled forced-air furnace fan. Check with the installers in your area for used fans; the cost may not be more than about $25, or they might even give you one. Cover the intake and all moving parts with heavy metal mesh, and run the power cord through a switch for easy on-off control.