To answer your first question ... of course, you can use Spring DI in Eclipse 3.6, but the current CSS style for Eclipse RCP UI was not introduced before Eclipse 4.
I cannot address if you should or should not create an RCP 4 Scratch project. This is a tough decision, and it should be done by someone with a deep knowledge of your project and its future goals.
As for directions and suggestions ... I have two sources of information for you.
Lars Vogella. His training sites blend very well, and I find them a must for anyone who wants to learn the basics of Eclipse RCP development. Unfortunately, these tutorials may not be detailed enough to help you. It depends on your current level of knowledge of Eclipse RCP. http://www.vogella.com/tutorials.html
Source Eclipse. This is a paid service. I never worked with these guys, but they sent me several letters offering several different levels of pricing for different levels of support. There the website blends well and offers excellent information. http://eclipsesource.com/en/eclipse/
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