Tam,
D17 is sort of a double edged sword. It is a slim doccument compared to some other welding codes which can be a real benefit, however, It does not speak directly to many specific craftsmanship issues because it has an expectation that if you are working on aircraft componants you have a some pretty extensive expertise. This can make it a challenge for folks in your situation.
Again, the standard practice manuals belonging to the OEM's for the engines you are overhauling ought to give you some insight into purge gasses.
There are 20 to as many as 40 different alloys contributing to an aircraft powerplant these days, some of those alloys are going to be overhauled with different procedures than when they were fabricated. There is no simple answer that will speak to all backpurging situatrions even though most stainless and superalloys will require argon backing on full pen welds.
There are forumlas for calculating purge gas for pipe that can be found in almost any welding textbook. ASM Volume 6 will be a big benefit on materials behavior also.
For overhauling liners and saddles, compressors, HPT, LPT boss replacements and other pressure vessles and there attachments you will need to fabricate your own custom argon backing fixtures, some will be integrated into your restraining fixtures others will simply fit the conture of the back of your componant.
Some of those argon backups will need to be plated to keep copper from migrating to the HAZ during welding operations (high cobalt haynes 25 being just one example of a copper sensitive material found in hot sections)....
You might also ask very specific material related questions here on the forum and get great advice too. But there is no simple instruction that is going to cover everything.
An expert consultant could set your quality program on it's feet and teach you how to keep it going.