19mm is 3/4inch in old money.Doesnt really make a lot of difference 19mm/20mm just depends on supplier.42m of 25x25x2 will be enough for any cockups etc (sorry mistakes
).ERW,Seamless,SHS again all have a seam on the inside so wouldnt worry too much.ERW does have a squarer edge though.
CTWV50 put your location in your profile as it may help in future.