if you want to do it right, don't buy anything. research first
u say a few decades before or after 1400, but let me tell you that is a WORLD of HUGE difference in armour, clothing, fashion and social. get down the dates to about 10 years or so. area (England, France, German) makes a huge difference on the same too!
i wouldn't buy either of those helmets. one they are way over priced and 2 they don't really look like the real thing
as to doublet. well before mid 1400s there were no "doublets." chausis and later hose were held up by the brais and then later "they think" by the pourpoint during fighting.
some good books for you to look into are the Osprey Publishing: Men at Arms or Elite or Warrior series. lots of different types focusing on short tidbits here and there, and cheap enough that if you don't really like that "look" you can get another book. another great book is 1381 The Peel Affinity. written and photographed by a local living history group. shows a great variety of armour and clothing for the 14thc.
here's just a few of my Osprey at easy reach of the computer, there are 100s of these books.
Osprey Publishing
Men -at-Arms:
Medieval European Armies, The Swiss at War, The Armies of Crecy & Poitiers, Italian Medieval Armies, German Medieval Armies, French Medieval Armies, Scandinavian Baltic Crusaders, Medieval Polish Armies
Elite:
Knights at Tourneyments
Warrior:
English Medieval Knights, Knight Hospitaller, Knight of Outremer, Italian Militiamen
for the correct way to do it, yes a hauberk needs to be worn over a aketon/gambeson in the 14thc but just for faires it maybe a tad hot heheh. again to the coif, research, the helmet you may choose might not call for a mail coif but a mail aventail attached to the helm itself
now don't get me wrong I love a great kit. whether its done historically, fantasy or just down right looks cool I just don't want you to buy something and then be ah man i could have spent this and got this instead cheaper or gotten this and looked better or more to what I want.
hehehe let us help you out here. find a great picture of what you aspire to be and we all here will do our best to help you look the most kick ass you can