Venture capitalists L. J. Sevin and Ben Rosen -- whose previous credits include Compaq Computer and Lotus Development -- have a new high-tech start-up in the gate. It's Benchmarq Controls Inc., in Carrollton, Tex., which manufactures handheld computers. "This is a very fast-growing market, very fragmented," says chairman Sevin, "and there are no real dominant players." Benchmarq's angle? The right price, he says. Handheld computers are used for fairly simple applications, he notes. Benchmarq's units are priced at $675 and up. More expensive models, Sevin contends, "are largely overkill for the problem."