The number one concern for UK authorities is that she can prove (as far as humanly possible) that she will return to Thailand.
A letter from an employer saying she will be away for a certain length of time (normal holidays in Thailand are not usually more than 2 weeks - so her getting 6 months off work probably won't fly).
Proof of ownership of house and land will help.
Having children in Thailand will also help.
A letter (and proof of income) from you stating that you will cover her expenses while in the UK.
A bank statement from her bank showing she has resources.
It ain't gonna be easy. You may have to resign yourself to visiting her in Thailand for a few years - or at least until you get married, even then it doesn't get much easier.
Good luck - lots have gone before you and found the whole process worth the effort.