Saildrive diaphragm every 7 years and as Martin and Sheena say, never heard of one failing but I am sure the insurance companies would use it as a getout if they thought they could!!!
I haven't put in a holding tank although there is bags of space in the enormous cockpit locker. I have several friends with holding tanks who have had serious blockage issues with them and I will avoid it until forced. Mind you, all these friends have Bavarias which have their own issues (no surprise there then) around the size of the holding tank outlet to the point where toilet tissue is banned aboard!!. We also use soluble toilet tissue aboard, just to be safe from blockages which we have never had, thank goodness.
I have had to buy a new exhaust elbow, rudder lip seals, water tank gauge sender and a new drive belt for the autohelm unit on the wheel. I had the compass swung as it was inaccurate. I also had the bottom stripped and epoxied/primed/antifouled in winter number one as it had never been done since new. Apart from these things, the boat has needed nothing in four years. I intend to renew the keel seal and bolts this winter as there is a small weep from the front of the keel joint which is annoying so check this on your prospective purchase.
I hope you get the boat and enjoy it, we have and will continue to do so!!!
Tim