Wena,
As utnik mentioned, you can't do impositions in Scribus, which you'll have to do using external applications. We had discussion about imposition before. Check
this thread for some infos. And, of course, don't forget to check out
Scribus Wiki.
It is easy to create an A5 document (as well as document of any size) in Scribus. Simply design your book as you'd do in any other app, by choosing A5 format during document creation process, setting margins, number of pages, two-side or one-side document etc., then defining master pages, styles and other elements. You can create an empty document with all things set, and use it as a template. When the book is finished, save it as PDF document, then do the imposition as explained in the thread I've provided a link to.
Also note that it is a good idea to divide your book into a couple of smaller parts (e.g., put every chapter in a separate .sla file–which is Scribus's native format), since Scribus might not work comfortably with documents larger than 30–50 pages. You can put PDF files together afterward (just google "merge pdf" to find utilities for your OS).
As for PDF/x-1a:2001, Scribus prior to 1.5.x doesn't support it (1.4.0, which will soon become stable, will not support it). However, current development version (1.5.0) does support PDF/x-1a, so you might consider installing this version. But be aware that it IS a development version, so I'd recommend that you use version 1.4.0.rc5 for your work until your project is finished, save the document (and a backup copy, since once you open and save a document with higher version of Scribus, you cannot open it again with lower version), then open it with version 1.5.0 and save as PDF/x-1a.