Welcome to the game and to the forum. 
A pet based Conjurer doesn’t need that many buttons to push once you’ve summoned them and applied auras. Have a look at Maya’s Carnival Guide to Pets and adapt it for the base game. Would probably need to get Mogdrogen’s devotion since neither Ishtak nor Tree are in the base game.
It’s up to you; you would need to buy the Ashes of Malmouth expansion as well since FG needs that. Both add more Acts and map content, new masteries, factions, items, quests, etc, but if you’re not sure yet you’ll enjoy the game then wait and play some more before deciding. You can always get them further down your play time and any current characters will carry over with no problems.