Poverty analysis and multiple linear regression

Hi. I have a dataset and I intend to use multiple linear regression analysis with excel since the dataset isn't that massive in volume, just about 500 rows. I have continuous numerical data and I already checked the scatterplots for linearity. Here are my questions:

1. I applied the model I came up with to the dataset to compare the actual and predicted values. How do I know the range of the acceptable errors?
2. How do I know if my model is already the best model?
3. Do I have to remove outliers?
4. How do I do the hypothesis testing for this?

Sorry, this is for my dissertation and I'm currently on break so I couldn't bother my statistics professor. Please just give me a walkthrough, I'm not sure where to start here. Thank you so much!