PLL/DDS Frequency Sweep


I am trying to make a small automated test equipment where i have a requirement to generate a sweep signal from 130MHz to 160MHz in steps of 25/12.5KHz for testing. The complete sweep time should be less than 5 seconds for the whole band. I tried achieving the same with ADF4351 EVAL Board, but it seems actual speed is limited by the usb and the step resolution by the loop filter value. i have several ideas but wanted to get some expert inputs on which approach is better in terms of ease of development without loosing accuracy.

1. What limits the ADF4351 to do the job. is it possible to make the sweep faster with 25KHz step spacing by changing the loop filter alone.

2. Will any other approach like using any DDS like ad9912 or 9957 help to make the sweep faster and accurate frequency  step.

3. What is the rate at which the dds can switch to different frequency, is it purely dependent on spi speed.

4. i am trying to avoid any use of mixer/upconversion techniques as i feel it make things more complex in terms of output spectrum.

5. if i use DDS, what is the simplest way to provide a relatively clean 1GHz signal.

Can anybody give hint on what could be the best approach/design with use of minimum components.