yes you can buy an onward ticket at the airport if needed, just go early so you have time if needed. The thai immigration don't normally check but that will depend on how often to hane been in thailand previously. If you have been lots of times with extensions then they will probably ask
I habe been many times and exchanged uk bank notes many times. They don't want marked or ripped notes or if they look like they have been crumpled up. However as long as they are in good condition even with some gentle folds in them, you will be OK. I have read many posts saying you need brand new notes otherwise you will have a problem, this is not the case. I typically travel with £20 notes as it's easy to carry, the exchange rate is only slightly better for £50 notes but it's typical only pennies. Just be sensible and take good condition notes
The DTV can only be applied for out of thailand not within the country so no agents can help. If they say they can it is not true. You need to apply yourself, suggest you for online to see the requirements in the country you are applying from
An agent will advise you to go for a retirement visa whether that is the best option or not. An agent cannt help with a DTV so will not make any money from you. A DTV can only be applied for outside of thailand so.no agent can help.