Nobody wakes up wanting a marketing agency. They want next month booked, and right now it isn't.
The usual response is to buy something vague and monthly and hope. Six months later there is a report full of impressions, a smaller bank balance, and the same quiet phone. The reason is almost always that the thing purchased was activity rather than outcome.
So the three things here are the three that end in a conversation with a buyer. Outbound calling produces them this month. Google Ads catches people already looking. SEO makes both cheaper over time. Everything else an agency can sell you is downstream of those, and most of it is decoration.
If you can only afford one, the honest answer is usually lead generation, because it is the only one that produces a conversation in week two.