I have a different approach. I always start off with the Plant Wall, level it up as far as I can afford, then get the Wolf Pet and level it up as far as I can. Generally at that point I will buy the estates upgrades, then the library and finally the extra life.
I always play Fantasy / Very Hard. I've saved quest files where I basically used white / blue soldiers the whole time (I retired it at 256 - 0) and another where I started with red / black goblins (that then morphed into blue milling and then morphed again into a 5-color Green
Door to Nothingness /
Helix Pinnacle alternative win conditions deck just to try out different deck types) that I retired at 118 - 0. My current deck is a white / blue milling deck with a 20 - 0 record.
Initially against the level 1 / 2 opponent decks, a good strategy is a quick weenie rush. Typically with 1 - 3 mana, high-power creatures, you often should be able to quickly beat down the opponents before they can even get started. My goblin deck was generally getting the turn 10 / turn 15 bonuses, which really helped being able to buy the cards that would help my deck. I sell
everything I'm not planning on using, regardless if it really nets me anything just to get it out of my way in the deck editor view. I wouldn't sell big ticket cards (over 5000 credits) until you have at least 50 wins to get past the 500 credit limit on selling cards, though.