Check your local plumbing codes. Most say no horizontal run over 5 feet without adding a vent to the roof at the washer. The vent will help prevent overflows. The airgap is an absolute must in most codes. Install lint traps if you are worried. The correct slope is a quarter inch per foot.
It is very common for laundries to be upstairs and close to the living areas. A pan wouldn't hurt but not required. The stainless steel flex lines are now being used as a better choice over rubber lines. Rubber hoses need to be replaced over time to avoid problems with bursting.