FYI the ‘flower market’ – it is in fact a plant market – at Chatuchak is only open on Wednesday and Thursday.
If you intend to stay in the Phaya Thia area, I would recommend flying into BKK and then getting the Airport Rail Link train (at 40BHT) which will take you straight to Phaya Thai station. Its far less hassle and expense than trying to get a taxi from DMK.
Getting to the market is also easy from the Phaya Thai area. Take the BTS from Phaya Thai to Mo Chit (4 stops) and then it’s a couple of minutes’ walk from the BTS station to the market.