The UK's recent decision to leave the EU makes this a difficult question to answer. It's still uncertain when exactly this decision will be implemented, but as it stands, you should be able to enter Portugal freely and as an EU national you'd be able to apply for residency from within the country (non-EU nationals aren't allowed to do this and have to apply from outside Portugal at an embassy or consulate). As an EU national it wouldn't usually be necessary to apply for residency (though it's always useful to have that kind of document), but in this case I would think it would be advisable to do that as soon as possible. There also may already be other effects from Brexit, so it would be a good idea to consult a specialist to get the latest details and the full picture. In the meantime, have a look at our Moving to Portugal and Visas for Portugal pages for some useful information.
Best of luck.
This largely depends on your lifestyle, personal preferences and standard of living, all of which differ from person to person. To get an idea of what a month's living expenses might be, make a list of what you would ordinarily buy in a month, including things like utilities, internet, transport, food, entertainment, etc. Then you can use online resources, such as the table at the bottom of our Cost of Living in Portugal guide, as well as well websites like Numbeo and Expatistan, to fill out these basic monthly costs to reach a rough total.
Bear in mind that the cost of living is always slightly fluctuating so any figures you come across should be taken as an estimation rather than a firm number.
Best of luck.
As non-EU nationals, you would need to apply at a Portuguese embassy or consulate outside of Portugal (so one in Thailand should be fine). For more information, have a look at our page on Visas for Portugal.
Wishing you all the best for your application.