DETROIT (AP) _ Ford Motor Co.'s purchase of United States Leasing International Inc. of San Francisco represents prudent use of cash reserves and also provides a ready customer for equipment, automotive analysts said Monday.

The agreement to purchase U.S. Leasing for $512 million, announced over the weekend, will add a segment to Ford's financial services business, said James Ford, chairman of the company's financial services subsidiaries, which include Ford Motor Credit Co. and First Nationwide Financial Corp.

''Ford is looking for more growth in that area,'' Ford Motor Credit spokeswoman Lynda Cummins said Monday.

U.S. Leasing, with assets of more than $1.5 billion, leases business equipment, automotive fleets, electronic equipment, and aircraft and railroad equipment.

That will complement the mortgage and credit card services of First Nationwide, acquired in December 1985, and Ford Motor Credit's services in financing for automobile dealers and customers, Cummins said.

Ford Motor Co. reported $9.1 billion in cash reserves at the end of the second quarter. Third-quarter figures are expected later this month.

''Ford has a tremendous reserve of cash that it wants to use,'' said Chris Cedergren, analyst with J.D. Power & Assoc. in Westlake Village, Calif.

Ford apparently is expanding with an eye to avoiding some of the initial problems General Motors Corp. experienced in absorbing its Hughes Aircraft Co. and Electronic Data Systems Corp. acquisitions in recent years, Cedergren said.

''Ford is buying companies more closely related to the heart of its business,'' he said.

Ford has recently announced acquisition of Aston Martin, a small English luxury car manufacturer, for an undisclosed amount; Hertz, the mammoth car- rental company, for $1.3 billion, and U.S. Leasing.

Cedergren called the purchase prudent because ''leasing is growing in the commercial sector.''

In addition, he said, ''Ford Motor Co. can benefit from selling them equipment.''

Under the agreement announced by Ford and U.S. Leasing, Ford will purchase all outstanding common shares of U.S. Leasing for $68 in cash. U.S. Leasing will remain based in San Francisco, where it was formed 35 years ago.