There is no such thing as a budget 928 but a budget for a 928. Get as many reference books as you can about the 928 so you are better prepared as a 928 buyer. This is the granddaddy of all 928 texts. Shows all 928 development beginning with sketches and clay models, to test mules and prototypes, punishing dyno tests, grueling road tests, a historical time piece and testament to the 928. Any dedicated owner will have these, if only to be able to converse with their mechanics on problem symptoms.
Used sets are available as well via the internet.
Make sure which set you are buying as the earlier set contained less volumes and does not cover the S4 and up. A cheaper alternative to the 9-volume set is to purchase the factory microfeche films and a used reader. And no, the 928 manuals will probably never make it to CD, at least not this century. Only covers up to 1981 I recall, but enough to do simple procedures. Heck, you owe it to yourself to try to arrange test drives with cars both in and out of your price range so that you get an idea of the entire range.