Not so much a guide, but a clarification.
Strongly recommend you go for this during your 1999 model run, as you will have to repeat 1999 mode to do this otherwise.
Notes:
- If you forget momentarily and accidentally press X by a Dollar Bill machine (perhaps when possessing it for cash and collecting the Silver Eagles) DO NOT WORRY. As long as you physically haven't bought anything, you'll be fine. Looking through the menu doesn't void the achievement. If you did buy something, I can't guarantee that loading an old chapter on 1999 works, but I don't see why it wouldn't.
- Veni! Vidi! Vigor! machines are 100% fine to use. These are for vigor upgrades. Buy away, just remember to prioritize upgrading Return to Sender and Devils Kiss to make the 2 hardest fights monumentally easier (see the superb Auld Lang's Syne solution by another player)
- Minutemen Armory machines are 100% fine to use. These are for gun upgrades. Buy away. Just remember to prioritise upgrading your favourite weapons and don't waste cash on guns like the pistol or machine gun with low DPS.