You've set a tall order.
Living aboard a small boat takes a special sort. Lack of storage is just part of it. By the sounds of your climate, you're going to need some serious cabin heat in winter (unless there's a typo there) and few smaller boats will have that.(but you can add it)
As to the holding tank, few boats of that vintage came with holding tanks, but many have been retrofitted, so you could too - at the expense of storage space, of course.
You may need to reassess the amount of stuff you "just need to have"
As far as to engines, there are thousands of A4s around, parts are available, they are smooth, quiet and powerful. Diesels are noisier, rough running in small sizes, but provide better resale potential as the market and insurance carriers favour diesel engines, primarily for the safer fuel storage situation.
Either boat will be a challenge to live aboard, but certainly people have done so with less.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)