The reality of $1000 cars
Buying dirt cheap cars off pages like Marketplace or Gumtree to build into project cars is a time-honoured tradition for many of us with undiagnosed problems. The challenge of buying a cool car for $1000, or turning a $1000 car into a cool ride, is a strong one we all feel the tang of time time to time.
While the siren song of Marketplace and the allure of dropping a mere thousand-bones onto some biohazard nugget is so strong it seems tied to our DNA, there are some stark realities you need to confront if you want to come wading into this murky water.
It should be obvious to anyone capable of critical thought that none of these cars are going to be ready to drive across Australia or make make big-banger power outputs. Any of these cars are projects and will need bulk time, money and elbow-grease to make into viable road vehicles once more. If that doesn't scare you off, welcome to Club Sick...

RUST VS ELECTRONIC NIGHTMARES
You can find some seriously cool old school cars around for $1k, but $1000 classic cars will have rot. No ifs, buts, or maybes. And not just chrome bumper classics, either.
However, rust is a nightmare across all socio-economic reaches of car enthusiasm. You could spend 100 times more than this and still be dealing with rust in some way - it's just likely you'll be dealing with far, far less than in a car costing a grand.
But are you scared more by rust or electronics issues in modern cars? There are plenty of late-model cars for sale at this price point featuring one, some, or a litany of electronics issues. Some have been drowned in floods (avoid at all costs), while others lack maintenance or are from brands and models infamous for crapping out expensive sensors.
You need to ask yourself if the solution to this car's problems was easy why didn't the former owner just do that and sell the car for heaps more mooah? Do you have the time, tools, or money to diagnose what is wrong? Faults in the ABS systems or buried in the body wiring loom can be insanely difficult to fix.
That said, I helped a friend buy a rare 1980s Maserati that had an engine bay fire for almost beer money. Several weekends of cleaning the bay, replacing hoses, and gutting the interior to replace the trashed wiring loom with a good 2nd-hand unit from a wrecked car had the old twin-turbo V6 back on the road for not much more than this budget. (Being an 80s Maserati it was almost instantly back off the road having broken down, but that's not the point of this story).
A final note on rust is that it hides, just like an electrical issue. A paint-depth gauge will warn you of inch-thick body filler, but the only way you'll know if this old nugget is truly rust-free is to de-skin it (remove all outer panels).

REGO
You're not getting any. No, none. Forget it. So, why is it unregistered? Does it have mechanical issues and, are they simple or catastrophically difficult to cure (like a dead engine or trans)? And, if they're easy to fix, why is the seller offloading the car cheap instead of fixing them?
Can you afford the cost of rego? Almost all cars in NSW will cost more than the purchase price to put a full year's rego on, and this might be a reason why the car is being sold unregistered, and could point to whether it has had any maintenance money spent on it during the former owner's time with it.
You also need to carefully inspect the car, including all the legal ID numbers for the car (and check them on a PPSR database). Is the car or engine stolen? Don't spend $1000 buying a car you legally can't own.

MISSING PARTS
There will be plenty of promising buys online for $1000 because the owner didn't know how to fix them, or have time or the money to do so. Can you? Maybe.
You need to give the car a close inspection to see if the missing parts have been hacked out using nothing but rage and the bite-force of a foul-tempered Doberman called Gerald (and I mean, fair, wouldn't you be pissed if you'd been named Gerald?). Sometimes what appears like a few missing trim panels or electrical connectors could hide a loom that's had parts ripped out with scant regard for anyone wanting to put the car back to running, driving condition.
Similarly, try to see if the engine starts, or at least turns over. Is there a transmission or a differential under the car? I was nearly suckered into buying a car which was promised "drove when parked" until I actually jacked it up and discovered there was no transmission, flywheel, clutch, or tailshaft - this made the car not worth the asking price or the seller.

MILEAGE
Whatever you buy will have likely the same mileage as if it had been driven to the moon and back. Weekly. That doesn't mean it is ready for scrap, as plenty of well-engineered base cars can go hundreds and hundreds of thousands of kilometres on their original mechanical hardware.
You should perform a few additional checks to see if the mechanicals are healthy, the body shell isn't too fired (do the doors and boot or bonnet open and close easily?), and does it need major suspension components entirely replaced? If it's done a few hundred thousand clicks (or more) it should have had all of its shocks and bushes replaced a few times, and probably control arms too.
Other parts which do wear out over time include door locks, the ignition barrel, body weather sealing rubbers, window guides, motors, and seals, as well as engine bearings and transmission and differential internals (bearings, rings, idlers, and more).

ECONOMIC VERSUS STATUTORY WRITE-OFFS
One reason you need to be careful checking numbers on these sorts of cars is because you don't want to build a sick project car out of something listed as a statutory write-off. That means it can never be registered again in your state. Dream, over.
An economic write-off will be listed as "repairable" on state paperwork. Any VIN (Vehicle Identification Number) marked with a "Statutory" write-off is not allowed to be re-registered even after a thorough inspection, as it is deemed as being "unable to be repaired properly".
You also won't be able to insure the car, so even if you're building a race car there's no coverage if it gets nicked from your house or is destroyed while being trailered to the track (unlike Laid-Up Cover from some insurers like Shannons). Never say never - this is why we have insurance - so it's something to bare in mind before you spend all your time and money building something that you will also be unlikely to be able to sell easily.