Yes, these kind of offers that guarantee you certain items are worth the gems, i’d say. They don’t show up that often though…
Good idea to focus on heroes now. Troops will get more important later in the game. But to that point food will probably not be that much of an issue anymore (well, of course you will be a bit short on food when leveling troops, but also be able to fill it up again faster). Maybe you will also be lucky and get some more mana troops before you start leveling. But as you have already 4* troops in every color, no more need to spend gems there.