Regarding the internal spacer, if you do use cartridge bearings (the correct term for what you're hoping to fit - cup and cone bearings can also be "sealed") you will need something to prevent whatever locknut you use from pressing the inner race of the bearings towards the inside of the hub. Most cartridge bearing hubs have shoulders machined onto the axle for that, but you can also use a spacer tube like the ones in the BMX press-fit BBs I work on. A piece of aluminium tubing in an appropriate diameter would do the job.