The easiest way for a foreigner to make money in Thailand is to work online and don't tell anyone.
I certainly wouldn't be starting a business within the first 2-3 years until you understand the lay of the land and have some decent ideas. Don't be one of the guys who blows all his money and ends up back in the UK working until he's 70
Legally, you're going to find it pretty hard to get a proper job.
You need to find a way to have an income from overseas. You wouldn't be the first person to be in Thailand working on your computer at home making money.
no need to mention she's pregnant, could go against you.
If she has it in the UK on a tourist visa they will give her healthcare no problem but she might get a bill at the end of it which will have to be cleared to get another visa.
Done it 4 times, including for the girlfriends daughter. It's not hard. Success rates are published by the Government and it's about 95% that are successful. Don't listen to all the people telling you it's impossible or hard because it isn't. You just need to get all the documents together and the evidence you provide needs to match up with what you write on the application form. You should not mention things that you can't back up with paperwork.
You can message me if you want any tips. Don't pay an agent, if you do, make sure they are registered with the government and not a backstreet chancer