Suntec Power Holdings Co., one of the world's largest solar-power companies, agreed to buy MSK Corp. of Japan for as much as 300 million US dollars, as the biggest acquisition by a Chinese company of a Japanese firm, said the company in a statement.   In the first step of the acquisition, which is expected to close in the third quarter 2006, Suntech will acquire a two-thirds equity interest in MSK for $107 million through a combination of subscription to new shares and purchase of existing shares. The balance of the consideration is based upon revenue targets for MSK and will be paid in the second step, which is anticipated to close by the end of 2007.   ''We anticipate that this acquisition will give Suntech the advantage of MSK's nationwide sales and marketing platform in Japan, which we expect to leverage to grow our market share in this important market,'' said Dr. Zhengrong Shi, Suntech's Chairman and CEO.   Although the deal's value is relatively low, it is the latest indication of stronger business ties between China and Japan.