You can buy ubolts from various auto parts stores. For all 8 of them, it should be less than $80.
As long as your F150 wheels are 5 bolt and off a 96 or older, it should have a 5 on 5.5" bolt pattern (which is the same as the samurai). For the center hole diameter, it has to be big enough to fit over the lockout hubs (be sure to measure the hub where it bolts to the wheel hub, that is where the lockout hub is the largest). Generally, 4 1/8" to 4 1/4" is the range that the holes are enlarged to.
You can post a "wanted" ad in the classifieds section on here, looking for tailgate hinges. Or you can check with some of the various suzuki vendors that part out samurais. You might even still be able to buy them new, from the dealer, if you want to go that route.
The tach cluster you need will come from an 86-88 samurai with the round vents in the dash. You can also just add an aftermarket tach, if you want.
Lower gearing would be good. Either in the axles, tcase, or both. If you go with lower gearing, you should be able to just run a stock clutch and it should hold up fine.
For flare options, that may be difficult, if you don't have anything to attach them to. You could try sticking them on, with some super strong adhesive. That may work better than trying to screw/bolt a flare or bracket onto really rusty metal. If the underside of the wheel wells are still in relatively good shape, you could get some rubber flares that just screw onto the underside of the wheel wells.