我要去中国means I'm going to china in a general sense；and 我要去中国了 means I'm going to china right now or in a very near future

我要去中国了。Or to make it sound a little bit more sophisticated：我即将启程前往中国。 If you wanna say "I'm on my way to China", it is "我正在去中国的路上“ or "我正在前往中国的途中”

欢迎来中国

yes, it's 我 要 去 中国。

You are absolutely correct.