I've had diesel engines on all of our bigger sailboats, & while they are of course very reliable, they are also very expensive to replace, & there are newer alternatives out there.
If there is a reusable engine block & majority of the engine is intact, then I would always rebuild an existing engine before replacing it. Unless it had a cracked block or head from extreme overheating, or excessive corrosion to the point that the cooling jackets were leaking. A rebuild will usually come in at about a quarter of the replacement cost. I feel that the C 30 is too big & beamy for an outboard engine to effectively push, unless its a big one, & that will weight down the stern transom excessively. The motor would also have to be offset to one side because of the swim ladder, that will effect its efficiency & cause the boat to track to one side. Plus I had a C 25 with a tiller & an outboard & its a ***** to steer & run the throttle & shifter at the same time when docking.
Another possibility is to look into the new electric motors that are out there. There are several past discussions on this C 30 website that detail replacing an old A4 for a silent electric motor, that uses no fuel & does not pollute. While they do need a lot of battery bank capacity, I do believe that this will be the wave of the future in terms of providing auxiliary power for sailboats. Diesel fuel is getting more expensive, it is stinky, creates pollution, and if I were given the choice over spending over 10k on a diesel replacement or an all electric motor that was silent & pollution free, I think I would go all electric. Just my 2 cents.