There isn't much info out there about the company, so don't bother digging too much. There are a few BF members with Marus, and the brand shows up on ebay infrequently. AFAIK Maruishi now goes by the name of Jamis here in the USA. The catalog I have is for 1985. Re the brakes, try Kool Stop pads or similar. Dual-pivot calipers would probably perform better with a load. Those in the Touring forum are best to ask about attaching racks, etc.

Can you post some photos of your bike? What kind of tubing does it have?