That's a good strategy that many use to spread the cost over time. The Manfrotto will serve you well up to 200mm or so, and higher with extra care.
I recently tap tested an 055 alloy out in Blackwater NWR. The owner had a Nikkor 80-400 VR attached. I was impressed with the tap test (at 400mm), considering the setup was beyond what I recommend. It did better than I expected.
You can save a little with the 055XB model, which may be a bit more stable than the PRO model and goes low to the ground without having to articular the center column.