Thanks for your inquiry on Agile Exptress Checkout module.
1) Can the country selection be limited?
You can set the countries to active or inactive at back office as store admin, this is prestaShop basic function.
Our module get country list from there, so it is possible for you limit the countries.
2) Can the shipping options be customised?
I am not sure if I understand your question correctly.
You can configure shipping carriers at back office Shipping - Carriers tab.
Our module get available shipping carriers from database your configured.
PrestaShop provide flexible carrier configuration function. You can configure different shipping fee for different zone based on order amount or total product weight.
3) Can the colors be altered to fit our client\'s website?
Yes you can modify the related theme file (.tpl) to meet your needs.
4) Can the text information be changed/configured?
Yes this can be done via back office translation function.
Or you can modify in theme file, but it is recommended to use translation function.