I wouldn't hesitate to buy the boat with a composting toilet, but it is a polarizing issue. I wish we could have put one on our Hunter 31, but none of the commercially available versions would fit, and I was too lazy to make one. The extra storage space will be nice to have. Folks that have never had a composter think they are messy, and stink, and attract bugs. If you use it properly, none of those things are true.
What is messy, and stinks, is dealing with the standard toilet setup when a problem arises. The hoses are difficult to route and connect, pumps need to be lubed and/or rebuilt, and if you have an overboard discharge for offshore use, macerator pumps go bad (and are connected to more hoses).
As to resale value, if you mean when YOU go to sell the boat, I wouldn't factor that in at all. A mid 80's C30 has already depreciated as much as its going to, so condition and equipment (and I mean the kind not related to the toilet) will make the difference between getting $6K and $12K. Buy the boat at the right price, enjoy it, and don't worry about what you will get out of it. This is especially true if it is your first sailboat.