To use as a pump, you do not need a CGM.
To use as Hybrid Closed Loop, you need a Dexcom CGM that can talk to the OMniPod
The questions you are asking about the advantages of pumping over pumps, is basic pumping information and not specific to OmniPod.
If you want to get the most out of your funds, I recommend research into pumping and hybrid Closed loop. There is training available from Diabetes UK and NHS such as this
https://elearning.mytype1diabetes.nhs.uk/courses/my-insulin-pump-the-online-education-course/
You do not need to fill the pump completely although you need to have some overhead in case you are unwell or eat a more carby meal in the three days.
Some pumps allow you to extract the insulin from the pump but you may need to be careful with reusing this in your next pod (assuming it is possible with OmniPod) as you will be mixing older insulin. I prefer to partially fill the pump, especially as pumping uses less insulin.
If you go ahead with self-funding pumping, I highly recommend factoring in the cost of a private endo to help you set up a pump as there are many things that you can tweak to get the biggest benefit.
I also recommend working out what you want to get from a pump to make sure it will give you the biggest value - for example, if your problem is a challenge with carb counting (I am not saying this is the case for you, it is just an example), you still need to carb count with a pump, it will not solve the problem.