I'll address the paint issue- acrylics vs enamels. I have modeling friends of both persuasions. I prefer enamels myself as I have had paint dry in my airbrush and had a devil of a time cleaning it.
I'd go with what you are familiar with. Unless the decals are very small, you will want to overcoat with a clear to seal the decals. And, you cannot apply decals that well over a flat paint. So one scheme is to paint with gloss, decal, and then clear flat.
But- don't make it too flat. Unlike naval ships, civil ships were not actually painted flat. They were painted gloss, but being exposed to the sun and weather 24-7, the paint chalked and weathered fast. A first class ship like the Titanic would not weather to an extreme- they tried to keep it as clean as practical, so an eggshell (semi matt) would be very appropriate.
An alternative would be to paint with flats, then apply a gloss overcoat just in area where decals go, then overcoat with flat.
As I say, an eggshell semi-matt might be more appropriate than flat (remember, the Titanic was a fairly new ship). One option for a clear overcoat is polyurethane varnish, which is available in a semi-matt.