Stud sensors do go along the lines of good - better - best, but it can be still very difficult to locate the studs due to the metal and mesh used in the walls. You best bet is to use a little better quality stud sensor, look down along the length of the wall for for slight variations where the studs might be, then tap around a little with a screwdriver handle, listening for slight differences in sound. When all three seem to come together, mark the suspected location with a piece of masking tape. Then use something like a 1/16th inch drill bit to drill into the area - do you feel a stud or nothing at all? Did you get a little, or a lot of wood on the bit? Home in on it by going side to side. The small holes can easily be filled with spackling compound afterward and are hardly noticeable - a little touch-up paint - invisible. Be careful though since older construction is not always on 16" centers and I have seen many older walls built with 2x3s. I'd stay away from single stud mounts and just use plastic molding to run the cables down along the surface of the wall.

To follow up on mxyplx's comment, it is not at all uncommon to find "unusual" stud spacings in really old houses. My own house, the oldest portion of which was built circa 1810, has corner posts and "studs' around the window openings only. This leads to areas close to three and a half feet wide without any vertical framing at all. Since the corner posts are at least a full 6 x 6 inches and the window framing 4 x 4 with 1.5 inch boarding boards on the outside and 1 inch lathing on the interior, it's probably at least as strong as more conventional framing.

the real question is how much are you willing spend??? The stud finders today have all the bells and whistles. From alarms to laser-levels to depth gages. What are you planning to do with it? Are you really worried about hitting an electrical line? Do you think there might be some hidden Steel Studs? There are Stud Finders on the market which can sense all of these things. But a good old fashion stud finder for $10-$15 bucks I think should be fine for things like hanging shelves. Or maybe get a "Laser Self Mounting" finder for around $30-$45, for hanging pictures all in a row, or hanging a mantle piece. There is a lot out there, or you can still tap the wall with a hammer. A few times of doing that, you'll probably won't need a stud finder.(ha ha ha)

If it were me, I'd use the old fashioned $3 stud finder which has a little magnet that you can observe: when it passes over something metallic, it swings visibly. Since the fasteners holding down the subflooring are nailed into the joists, locating the old fasteners should show you where to put the new ones. Further, once you locate one of the joists, the others should be parallel to it, at regular intervals (often 16"), and you can pretty quickly mark the locations with masking tape, a yardstick and a pencil. Use screws and drive the heads just below the surface, fill them in with floor leveling compound, and put new vinyl down. Unless you have had the old flooring tested to ensure it doesn't contain asbestos, don't tear it out. You can put down a new layer on top of it unless the old one isn't well-adhered. If the old flooring material is not well-adhered, and it tests positive for asbestos, then follow the repair procedure described above, lay down another 1/2" of underlayment which you can screw and glue down over the old flooring, and then put down new vinyl.