Typically, repeater is used to extend distance and to boast the signal strenght. And its not cheap. Thus for many wireless LAN, its is more feasible to use multiple access points but you need to have some cabling done for all the AP.
Whether to use a, b or g, my take will be to go for G since many AP/router in the market already supporting G. Will you want to use a old technology for almost the same price? Do also note that cable modem can goes more than 1.5mbps.
Just my 2cts
JC