After living in a small one-bedroom apartment in Houston for a year, Matt and Suzanne Stillings are ready to settle into home ownership. With a maximum budget of $400,000, they have decided to look for a home within Houston's ''Loop,'' preferably in Midtown. They want a spacious three-bedroom, two-bath home, and their wish list includes a yard, possibly a pool, storage for bikes, surfboards and watercraft and space for an herb garden.