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I've seen people say to only use a PSU while coding/flashing while others say it's okay to use a battery charger with the correct specs.  I already have a portable jump box and a large battery charger/maintainer/jumper but it doesn't have an AGM setting.  If possible I'd like to find a NOCO brand which can both supply the correct power to the car for coding as well as maintain and slow charge a battery. 

What amp rating do I need for this and is there anything else I should take into consideration?

NOCO products are currently discounted for Cyber Monday and if they have one that will work I was going to buy it today. 

For flashing you´d need an power supply that can supply at least 13V and 60A continuous current over long time.
The NOCO GENIUSPRO50 is the only charger that comes close to the specs needed.

would be much cheaper to DIY an flashing power supply

there is few availlable but not cheap Gysflash ,ctek and lemania all capable 14v 100A

https://www.amazon.com/Schumacher-INC100...1138&psc=1

or GYS FLASH

No other question.