@Jeffrey643 adding to @Helen3 's list
1) Check with the local government that your changes comply with fire regulations and/or any other building regulations.
2) The budget will be determined by how creative/hard working you can be. You can furnish a complete house on Facebook Marketplace/Ebay for a relatively small amount and you can achieve a stylish result if you set off with a scheme in mind.
3) Check with your insurance company and mortgage company that you will be covered for Airbnb usage. This really ought to be number 1 as many will not allow you to. Also Aircover doesnt work as its (poor) coverage only applies when guests are there and the fact you are Airbnbing usually invalidates your existing policy even when guests are not there or have not caused the issue themselves.