you can use the olympus lenses with the olympus 4/3rds cameras, however, you should ask yourself whether it's worth it. the old olympus lenses won't autofocus, nor will they meter at full aperture, and metering options on the camera will be limited (e.g., no shutter priority). in other words, they'll be a bit of a pain to use.

most slrs have instant startup, although the ones with dust shakers at power-on may take a second or so (and it usually can be disabled, or set to run at power-off or on an as-needed basis). olympus 4/3rds cameras are also not significantly smaller or lighter than the smallest nikon and canon cameras (only a few millimeters difference), yet the sensor is much smaller with no option for full frame in the future.

go to a camera store and see how the various cameras feel in your hand as well as how easy it is to manipulate the buttons and navigate the menus to adjust the settings. also look at the lens lineups and decide what best matches what you plan to shoot.