After spending six years building his computer consulting business into an $18 million systems and software firm, Robert L. Quinichett has decided it's time for a rest.

Quinichett agreed yesterday to sell his company, Sterling Systems Inc., to his McLean neighbor, Planning Research Corp., for an undisclosed amount of cash.

"It has nothing to do with money," Quinichett said yesterday. "I simply decided that it wasn't what I wanted to do any longer.

"I've just been so intensely involved in the business, I'd like to just back away. Even when you're not here, when you're running a business, it's always seven days a week, 24 hours a day."

Quinichett said he would stay on as president until PRC names a successor in a few months, and then will become part-time chairman for two years. Sterling Systems' 400 employes will continue on as the company becomes a wholly owned PRC subsidiary.

"The only person that would be replaced is myself," Quinichett said.

A former Department of Agriculture employe who had been a computer systems specialist for several companies, Quinichett started Sterling in 1977 and is its sole stockholder. The company now provides a wide variety of computer engineering and software services to government and corporate clients.

One of Sterling's biggest businesses is supplying litigation support to legal departments and law firms, doing computer searches of libraries and other sources for legal information. Last year, Sterling Systems had revenue of $16.2 million, good for 41st place on Black Enterprise magazine's listing of the nation's 100 largest black-owned businesses.

When Quinichett decided to sell the company, he said, he had a broker feel out several possible buyers, all of which were interested. But PRC's was the only offer that was seriously considered, he said. It is PRC's first major acquisition since 1976, a PRC spokesman said.

Both Quinichett and PRC Chairman and President John R. Toups said they expected Sterling to fit well into PRC's diverse mix of professional services offerings.

Quinichett, 46, said he has no plans for his new-found free time, although he doesn't rule out a return to business. "I simply wanted a little different style of life," he said. "I'd just really like to have my life unstructured for about six months."