Merlin vs Raptor
Merlin 1D uses gas-generator cycle (simpler, cheaper). Raptor uses full-flow staged combustion — the most efficient cycle ever flown. This is why Starship can be reusable.
Why Methane for Starship?
RP-1 (kerosene) is dense but leaves soot. Methane is cleaner, can be made on Mars (Sabatier reaction), and works better with full-flow staged combustion engines.
What Δv actually means
Δv is "change in velocity". To reach Low Earth Orbit you need roughly 9.4 km/s. Your current design has 0 m/s. Add more stages or better engines to increase it.