* Existing home sales data on tap
* Goldman cuts Boeing to sell rating
* Valeant, Salix agree to merger deal
* Indexes off: Dow 0.39 pct, S&P 0.25 pct, Nasdaq 0.05 pct (Updates to market open)
By Chuck Mikolajczak
NEW YORK, Feb 23 (Reuters) - U.S. stocks edged lower in early trading Monday amid weakness in oil prices, after the Dow and S&P closed at records last week in the wake of a conditional agreement by euro zone finance ministers to extend Greece's bailout.
Greece will present its economic reform plans on Monday to seal the euro zone financial lifeline, but the government drew criticism from a veteran leftist and ruling party member that the deal let voters down. The deal is conditional on Greece's European and IMF creditors accepting a list of reforms drawn up by Greece.
"In spite of Germany taking a hard line, the proposals will most likely be accepted by the EU and the reason for that is simply Greece cannot afford to depart from the euro zone, not because Greece is a big economy, but from a psychological standpoint," said Peter Cardillo, chief market economist at Rockwell Global Capital in New York.
"The EU does not want Greece to leave, because if (it does), it sets a precedent and opens up a new can of worms - who would be next?"
Oil prices fell, with Brent down 2.4 percent to $58.76 and WTI crude off 3.6 percent at $48.99 a barrel on oversupply concerns and a stronger dollar, pulling the S&P energy index down 1.2 percent as the worst performing of the 10 major S&P sectors.
The equity market gains had led each of the three major Wall Street indexes to their third weekly advance on Friday, with the Nasdaq on an eight-session winning streak as it closed in on the 5,000 mark for the first time in nearly fifteen years.
Existing home sales data is due at 10:00 a.m. (1500 GMT), the first in a flurry of economic reports this week that will help investors gauge the strength of the housing sector.
The Dow Jones industrial average fell 71.06 points, or 0.39 percent, to 18,069.38, the S&P 500 lost 5.21 points, or 0.25 percent, to 2,105.09 and the Nasdaq Composite dropped 2.59 points, or 0.05 percent, to 4,953.38.
Boeing fell 2 percent to $155.13 as the biggest drag on the Dow after Goldman Sachs cut its rating on the aircraft maker to "sell" from "neutral."
Canada's Valeant Pharmaceuticals International Inc agreed to acquire gastrointestinal drugmaker Salix Pharmaceuticals Ltd in an all-cash deal valued at about $10.1 billion, the companies said on Sunday. U.S. listed shares of Valeant gained 14.7 percent to $198.77 while Salix slipped 1 percent to $156.20.
Asahi Kasei Corp, a Japanese chemicals maker, said it would buy Polypore International's energy storage segment for $2.2 billion, sending Polypore shares up 12.4 percent to $59.53.
Declining issues outnumbered advancing ones on the NYSE by 1,613 to 1,132, for a 1.42-to-1 ratio on the downside; on the Nasdaq, 1,463 issues fell and 861 advanced for a 1.70-to-1 ratio favoring decliners.
The benchmark S&P 500 index posted 50 new 52-week highs and no new lows; the Nasdaq Composite recorded 72 new highs and 15 new lows.
Editing by Bernadette Baum