Are there general guidelines for deck building in terms of how many of each type of card to include? For example, MtG suggests so-many lands , so-many characters etc in a deck. Are there similar suggestions for WHI to get started with?

A quick look at deckbox.org or at the tournament decks will reveal that builds really depend on the strategies you are interested in exploring. For instance the current world champion used a deck with only 6 units because he was relying on a potent combo to win.

Past decks seem to have at least 20 units with most running 20+ without exceeding 24 or so. 

Many in the past have excluded quest cards, but recent developments in the meta game have people reconsidering them. 

At my gaming table, we don't  delve into deck construction but play a version where we take a faction, blindly shuffle in 23 units, 7 support, 8 tactics, 2 quests, and 10 Order or Destruction or neutral Neutral cards. This seems to work well and allows us to play pretty balanced decks that explore all the cards in my collection. 

Hope that helps.