Now that you own the boat, I'd check to see if it was winterized correctly for this winter. I'd wait on changing the engine antifreeze as long as it's good.

Same as the oil, it's nice to be able to warm the engine up. Check everything that may be damaged by freezing, seacocks, blige pumps, waste pumps, head, deck wash, ice box/shower sump, as well as the fresh water plumbing, and either check they're dry or use anti freeze. There's a lot of stuff that water can sit inside and freeze.

Having said that, I just changed my antifreeze this fall but I know the cooling circuit and that it is easily refilled throughout. When you do that, check the draining coolant for sediment and scale to see if you should flush it before adding new antifreeze in the spring.