A few years ago I seem to remember there were international flights into Agra, but none that I know of since then.
Best suggestion I think is to fly BKK-DEL, then take a taxi or car to Agra. it's an interesting drive! Many many agents in India will arrange this for you.
As to hotels I can personally vouch for the Trident and ITC Mughal but neither is exactly close to the Taj. Mind you anything that is close to the Taj will not come in at under $150. If you have brought a car and driver from Delhi, then the two or three mile drive won't bother you.
The Taj is certainly worth seeing.
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