there is no "best" hero. some heroes play well vs others and not so well vs the rest. if you dont intend to spend money, you can
kiss the hope of getting all gold cards goodbye (unless u play 8 hours a day). If so i suggest you disenchant the extra gold cards once u got 2 copies of the normal cards.
as stated in the previous posts, once u decide to craft a legendary, craft Dr. Boom first. but imo, its better to spend the dust on crafting the good rares and epics first. they will help immensely in clearing your daily quests.
take for example a druid. for the price of 1 dr. boom, u can get 2 force of nature and 2 ancient of lores, which are all essential components of a druid deck.
I disagree that there is no "best class" in the any given meta, at least according to tournament statistics. While true that every deck has its counter, there are certain decks which inherently has more favourable match ups than other decks. This is a reason why tier list exists, whether they are based on subjective opinion or backed by hard data.
From my own compilation of tournament statistics, mech mage is technically the "best" deck in the current meta. The sheer tempo it grants allows the mech mage player to gain control of the board and overrun the opponent. But as mentioned before, it has its own counters, though not as many as what most people perceive.
For newbies, it is generally advisable to go for a deck which is cheap yet competitive. I strongly recommend face hunter to a newbie, as much as the average player hates face hunters to the core. The inherent low cost of the deck allows the newbie to have a feel of how a tournament worthy deck works.
Regarding crafting strategy, several reddit posters have done the maths. It is generally better to craft legendaries first, as opposed to epics and rares. The reason behind such strategy is simply due to the fact that legendaries are significantly harder to obtain from packs.
TLDR: Mech mage is probably the most competitive deck currently, but face hunter is the cheapest competitive deck for a newbie. Always craft the required legendaries first before crafting epics and rares.