I think somewhere around 90% are registered, it will be hard to justify a buyout of the 0NNN.org's probably because these are always the weakest numerics. As for the others, I think they are probably good, depending on the numbers. 1535.org is probably much more valuable than 7565.org, unless there is something about the latter I am missing. 1NNN.com's sell for over $1,XXX minimum, whereas other NNNN.com's sell for around $300-$500 minimum, so there is a big premium on the domains that start with 1. This is because there are many, many more uses for them, just do some random google searches for 1NNN versus other NNNN numbers and you will see a big difference in results. Whenever any new 4 digit codes come out for anything, they will usually start with 1000 and go up from there so your 1535 probably represents a lot of different products and things in society that in turn represent potential endusers.
However, the BIG problem with NNNN.org's is that Chinese domainers aren't really that into .org's, and neither are Chinese surfers. So the traditional sector that drives demand for numerics, the Chinese market, is just not there for NNNN.org's. You won't get type-in traffic for NNNN.org's from China like you would for .com's and .net's, and Chinese domainers aren't going to buy them. But I think I did a survey of some 1NNN.org domains and found that a lot of these were developed websites actually, so despite the lack of Chinese presence, a good low-number NNNN.org can actually have a lot of potential uses and enduser potential.
Good luck!