I would say 18mm thick custom wood would be suitable as I currently have a stand made entirely out of 18mm.
Most important thing to note is that custom wood will sag, especially on the top if you have no bracing.
for a 4 ft long top I would ensure that you had 4 pieces of uprights in the frame. 2 sides and 2 in the middle to spread the weight. 4x2 in the front and back would not hurt either but I would have it rebated into the uprights so that it also has support.
To stop the customwood from soaking up the water I would recomment that you paint it really well to stop water from being absorbed into the customwood. Polyurathane will do the trick quite nicely. better still if you can get hold of the meltica coversheets for the top would make quite a difference.