Welcome to CF. Where in NZ are you?
There are a number of local sailmakers who can provide excellent and personal
service (I can personally recommend Dave of Calibre
Sails in Whangarei and JF of Nautilus
Sails in
Auckland, but there are plenty of others), several large companies that are more
commercial (Doyles, Evolution, Lidgard, North, etc) and there are online Asian-based sailmakers (Rolly Tasker, Lee Sails, etc).
Google is your friend.
Sunbrella material is pretty standard for UV strips. You can also consider getting a
furler sleeve for use when berthing, which you hoist on a separate
halyard over top of the furled sail to provide further UV protection.
For size, if your
boat has a standard L400 rig then you should be able to get the sail measurements from the sail plan, either for your boat specifically (did your boat have its yacht manual?) or for your
Lagoon 400 year model generally via
Google.
Material is totally up to you and your
budget, but generally Dacron is most suitable for cruising sails if durability is your primary concern. A
sailmaker can advise on cloth weights and construction methods. Most
sailmaker websites have information about all of this.