Depends a lot on what you want to use it for, and how much repair work you can do yourself (or are happy to pay others to do).
If you are looking for a daily driver, it's not a bad idea to find something with existing (not expired) rego. After more than half a century on the road, there is likely to be wear and tear in some unusual places. The registration authorities can be funny about what they will accept for new registration - many will not tolerate any form of rear main seal leak, some get funny about stuff as fiddly as torn seat covers. If you cannot do the work yourself, it is not unusual for the rego authorities to make demands amounting to $2000 of work to get an FB/EK out of a carport and onto the road. Having existing rego means you can do the work at your own pace.
Rust is present in any FB/EK... just better controlled (or hidden

) in some. Front (and to a lesser extent rear) floor pans, front subframe, boot floor, rear parcel shelf, front drain plenum, around the windshields, bottom of the front guards, bottoms of doors, sills.... plenty of places to find it. If you're handy with a welder, it's all repairable. If you're paying someone else to do it, you can rack up $5000 really quickly.
If you do not have registration, beware of anything that has had the grey motor swapped for a red motor in it. A common swap for decades, and historically acceptable to the rego authorities... until the last few years. If it has a red motor and rego, it is generally accepted as "grandfathered", and can continue to be registered. If it has a red and no rego, many authorities will want full engineering... think $2000-$5000 worth of changes and paperwork. The humble grey will take years of abuse, and run with almost no compression in it. As long a you don't mind adding some extra oil, they chug along for a long time. A motor that has been rebuilt in the last few years is a bonus... overhauling a grey will not give you much change from $1000-$3000, even if you are handy with the spanners. If it's only a weekend car, you can probably live with a half-knackered grey for quite a while.
The numbers above are pretty scary, and can make a $5000 bargain turn into a $20,000 moneypit pretty quick. If you want to end up with a showpony or have aspirations of a V8, add another $10,000 to that. If you're handy with a MIG, spanners and spray gun then you can do a lot yourself for less cash.
Cheers,
Harv
327 Chev EK wagon, original EK ute for Number 1 Daughter, an FB sedan meth monster project and a BB/MD grey motored FED.