Steward Health Care System on Tuesday unveiled its second deal in four days to buy a Massachusetts community hospital, saying it had signed a letter of intent to purchase Saints Medical Center in Lowell and convert it to a for-profit institution. But before the ink was dry, the president of a group representing about 40 primary care doctors and 160 specialists affiliated with Saints said he opposed the alliance with Steward. And the chief executive of rival Lowell General Hospital, which lost out in the bidding for Saints, said he would challenge the deal as state regulators review the proposed sale. The dissenting voices represent the toughest potential roadblock so far to the hospital network Steward has been assembling across eastern Massachusetts. Steward, based in Boston, was created by New York private equity firm Cerberus Capital Management to manage the six Caritas Christi Health Care Catholic hospitals it bought last year.