DAC8820: Suggestions required for a low noise, fast settling DAC for pretuning VCO in a PLL

Hello

I need some suggestions to select a fast settling DAC part which I want to use to pretune a VCO inside of a PLL loop for faster frequency switching time (< 1 us). I want to combine the PLL loop filter output and the DAC output in a non-inverting summing circuit which will output the Vtune for the VCO. In this regard I want to know

1. How the "Output spot noise voltage" specification of the DAC might influence the overall phase noise of the PLL? Is 10 nV/sqrt. Hz as in DAC8820 a low enough specs for this application? What is this spec for TLC7524 which has a faster 100ns settling time?

2. Is it preferred to switch the pretune DAC output to the loop filter output using a SPDT once the DAC is settled or combine them both in a summing circuit?

3. Will a high slew rate OPAMP e.g. THS4031 be able to meet the settling time specs. of DAC8820 instead of using OPA2277?

4. Any other TI DAC suitable for this application preferrably with a parallel interface? please suggest.

Thanks