You're fine as far as arriving that late...lots of flights do. There also won't be a shortage of taxis, though it's important to ignore the touts (go to the coupon counter) and insist they use the meter. You'd have to pay the meter + 50baht + road tolls; for me it's usually 250-300 baht all-in. Don't expect them to have change. Some drivers really try to fleece unknowing first timers, though it's better than it used to be. The trip will take only 30-40 minutes, so it's better to look for something intown. Rates have gone up quite a bit in the midrange, but any number of places in the low-numbered Sukhumvit sois are in the 1,300-1,700 baht range. You can go lower in some places, but it'll be a real contrast to the Hilton.