What they all said. Things to think about, from what you say this woman has a very good reputation in dogs. She is offering to mentor you though co-owning. This is as much for her protection as it is yours. Co-own or not, it's unlikely that the dog will come without a strict contract. Part of that will be a stipulation that dogs on limited registration MUST be spayed/neutered.
Co-owning can be good if you work together, a disaster if you don't. Ask her how long she wants to co-own, perhaps she would be willing to do it for a set amount of time or litters. She is covering herself so that she has a say in who the dog is bred to. Afterall, she knows what bloodlines will mix best with her dogs while you are admittedly a novice. Say you took the male and let your neighbor down the road breed his female. The litter is a mess, bad temperments, some end up in the shelters, whatever. Who is the world going to blame, yup, the breeder of that super-expensive male dog. Sad, but true.
As others have said, read the contract and have someone else with experience read it. If you have any questions, it can be written out in greater detail. Be sure that it is spelled out who pays for EVERYTHING. Food and basic healthcare is you of course, but what about healthtesting? You mentioned showing, so be sure those fees are discussed as well. For breeding, discuss who will whelp the litter (if you breed the female) and who will decide who buys the pups. Etc etc.
For example, while I don't co-own, my boy's contract has a breeding stipulation. The breeder is hoping he will grow out nicely so wanted the option to use him at stud. The agreement is that I pay for all health-testing and any titles I wish to pursue. The breeder will decide after that if he is worthy of breeding. If so, he gets 3 litters from him (or x number of years whichever comes first) after that I am free to stand him at stud myself, though that will be with my breeder's help because I am still learning. during the time of the contract, all the fees will go to my breeder. After, they will go to me. Or, as he put it, you own the dog, I own his sperm. At least until the contract is fulfilled.