You're a gentleman and a scholar and there's **** few of us left. I've been taking great pains on a very limited budget to slowly restore my entire '76 back to original configuration inside and out. I personally haven't seen an aftermarket doodad that makes the Z look better than it did coming off the assembly line — well, except for nice wheels and a Grant mahogany steering wheel. I will also do a suspension and brake upgrade as time permits. But the engine, body and interior will be stock.
How do you do it? How much money do you have? If the answer is "a lot", just take it to a reputable upholstery or restoration shop, give them a blank check and tell them, "Make it new!" If not, almost everything you will need to do it yourself is commercially available, from rugs to weatherstripping to replacement seat and door upholstery. Again, though, not cheap. For a ZX, a new dash to replace that cracked one will run you $300-$500. If you're on the low-bucks end of the food chain, wrecking yards are a great, though hit-and-miss, source. Plus from personal experience, I can tell you that when you find the interior parts you need, they will never be the right color... until you sell the car. Fortunately, vinyl and plastic paints are available these days and better than ever. You can also find a lot of treasures, some old, some still new in the plastic, by perusing eBay. Try keywords "280ZX", "Datsun Z" and "Nissan Z". Another method I used was to find a trashed (in this case, severely rusted) similar car with a better interior than mine and transfer all the good stuff over. Not sure about '83s, but in the first-gen cars, the seats will fit either side. Since driver's seats almost always wear and many passenger seats are still nice, I simply swapped the good passenger seat from the rusty car for my drivers. (You do have to screw around a bit to get the seat release levers right). I also salvaged a bit of carpet and the heater beazel and voila! — new interior. Well, MORE new than it was before. When you're done with the wreck, you can part it out and sometimes make most or all of the money back, depending on what you spent in the first place.
If you know someone who does canvas or upholstery, see if you can trade them something (some work you're skilled in, sweeping the shop, a date with your sister, etc.) for the varsity stuff like seats. Even if you have to pay, a pro can often whip out a really nice shift boot and handbrake boot for less than you'd pay elsewhere — and most of them have enough scraps laying around that it can even be leather.
If you're low bucks, set modest goals and do it bit by bit. Most people won't even notice most of the stuff you do, but you'll know. I finally found an excellent "280Z" badge that goes on my right dash for a few bucks and it makes me feel like I've transformed the whole inside of the car.