If I only spend a little, what should I buy first in Dragonfire?

If you decide to put in a small budget, here's a priority order based on how long the gain lasts (purely informational — the call is yours):

1. The Lifetime Third Builder (€9.99) — the best comfort gain: permanent, and it helps your whole progression every day.
2. Cores to finish a key Habit on a pivotal Legendary (especially the ⭐ set Habit that reinforces its Order).
3. A Star Rank jump on one single pivotal dragon (permanent gain, +20% stats if Legendary).

Avoid sprinkling: 3 medium purchases across 3 different dragons are worth less than one concentrated investment in your key dragon. The through-line: favor what's permanent (builder, stars) over what gets consumed.