Unless you're aiming for sound reinforcement where the levels are low and the audience is very attentive, it's often better to dispense with delays completely, and put your 'relay' speakers ten to fifteen metres down the room. The idea is to keep the air moving in one direction.
As Bruce said, getting them above head height is quite critical.
We used to do the main arena sound at Shrewsbury Flower Show, and we had a row of seven columns of four Bose 802s along the front, with JBL 4852s on scaffold towers around 100 feet into the audience (10,000 people at peak). As it was in the open air, setting the delays for the towers was fairly simple because there was no slapback, but it would have sounded dire without delays.
We used microwave links to feed the signal to the delay towers, so the only cabling needed was mains, courtesy of Manweb.
If you must use a delay, a cheap way of doing it (but effective) is the Behringer 'Shark'. Yes, it's singe channel (mono), but you should be running the system mono anyway... and if you don't know why, think about it!