Within 24 hours and leaving via an airport you’ll get an overstay stamp in your passport but they’ll usually waive the 500 baht fine.
The overstay stamp isn’t an issue for returning to Thailand but can cause issues in other countries when applying for a visa for there. They see the overstay stamp for Thailand and may it cause problems.
That depends on the embassy you use. Should be around the 10k baht or maybe a couple of thousand more - unless you’re applying in NZ where it’s approx 40k.