How big a job it is depands on what fixing it means to you. As mentioned in other posts, doing an annual cosmetic cleanup is not too onerous, just understand that the keel back in the water rusts again almost immediately leaving you with a high-drag appendage.
If fixing the keel means having the equivalent of a lead keel, the price is a prep job of perhaps 20-50 hours to grind down, prime with epoxy, fair and sand, and then cover with another 4-5 coats of epoxy. Do a good job and don''t hit too many rocks and the keel job will be good for many years.
I did a full job with the West system products on a Rhodes 19 (they only come with steel keels) and was very happy with the result, for perhaps 20 hours of work.
I also had a Beneteau First 30 with a steel keel which only got annual patching, and I said "never again" and have avoided the same ever since.
IMHO if the prior owner didn''t deal with this situation, you should expect that he/she didn''t deal with other maintenance issues which may not be so apparent - if the keel bothers you, you probably should move on as there are plenty of quality 23 footers that didn''t cut corners and at this point don''t cost much more than boats that did.