I'd second your choice of an alternator. When preparing for the ARC it was the most common failure with at least 3 boats needing replacements.

Spares for the head are vital. If you have a Par/Jabsco type consider a complete pump assembly.

Do you have a pressurised water system? If so, can you get water out of the tanks if the pump fails?

It's always a good idea to stow bottled water in addition to the tanks.

I fitted a second set of nav lights and have never regretted it - 10w when sailing, 25w when motoring and a spare instantly available if a bulb goes.

As many ways as you can muster to get juice into the batteries. Nothing spoils your day as much as a flat battery (except perhaps a hole in the hull).

Assume your steering will fail and plan for it. I once had to sail 800 miles with a broken quadrant which wasn't funny.

I have big windows. To cover the possibility of one getting stove in I carried a couple of solid bars with bolts and wingnuts to secure a bunk board over the hole.