For broker's fees the nominal rate is 6% of the sales price but most will agree to 5%. You are as well off going with one of the majors like Corcoran or Douglas Elliman as anyone else. Brokers are ten a penny in NY and they are all pretty much the same.
Certain brokers tend to specialize in individual buildings. Search for your building on Streeteasy.com and it will show a list of brokers who have sold units in that building. Streeteasy is also good for researching price history and is worth the $10 a month subscription for full access. Atternatively you can search public records for free through ACRIS on the nyc.gov website (google it) to see price history for similar units in your building.
Attorney fees should run about $1,500 to $2,000 for the closing, plus another grand or two for various title, bank and bank attorney fees. There are also state and city real property transfer taxes of 1.825% of the sales price to pay. Not sure about capital gains tax for a non-US taxpayer though.
Good luck.