Montgomery County received proposals yesterday from two investment firms interested in buying 33 of its Ride-On buses in a deal that means tax write-offs for the purchaser and $400,000 for the county.

Under the plan, the county will sell the buses to investors who can deduct the $2.9 million purchase price from their taxes over the next five years.

The deal is possible because of a special provision of President Reagan's 1981 Economic Recovery Act that was designed to help beleaguered transit systems. Montgomery County owns 155 Ride-On buses, but only the 33 it purchased last year were eligible for the plan. Several jurisdictions across the country already have taken advantage of the sale-lease agreements and Montgomery plans to use the funds to help offset an expected $4.7 million reduction in federal aid.

County Finance Director Albert W. Gault said the firms that met the 3 p.m. deadline yesterday were Dean Witter Reynolds Inc. of New York and Baker Watts & Co. of Baltimore. Gault said Dean Witter's proposal was for $420,000 while Baker Watts' bid ranged between $379,000 and $437,912.15.

The only money that will change hands will be the $400,000 or so contained in the proposals, Gault explained. The $500,000-plus in interest the buyer pays the county in "paper transactions" over the next five years will be negated by the cost of leasing the buses back to the county for continued use. At the end of the five years, the county will buy the buses back for $1.

Although some members of the County Council said they were philosophically opposed to the plan, which was proposed by County Executive Charles Gilchrist at Gault's suggestion, it passed unanimously.

Gault indicated he will decide today between the two proposals. The decision must be advertised over two weeks and the deal must be made final by March 1 to allow the purchaser to take advantage of depreciation this year.

Jeremy L. Ragus, an associate with Dean Witter, said such ventures can be a bit risky for the firm that wants to buy the benefits.

" A firm has to have taxable income or they will lose money on this. They also run the risk that tax laws might be changed again," Ragus said.