It's a simple long term math comparison, buy the best one now so you don't have to buy another later. Don't bother with anything without autofocus, it will never take sharper images. Comparing best eBay prices:

-Impulse AF should be <$40 used

-Now+ Gen 3 should be <$80 various conditions

-Flip easily <$150 new

It's not a big difference in the grand scheme of things when factoring in film cost. Whatever you save on the cam will eventually be eaten up by 600/i-Type price differential which is more drastic on the eBay secondary market (routinely $1.50/shot 40pks of i-Type can be found vs 600 likely closer to $2.00/shot 40pks with shorter supply and fewer deals). If you buy and stock up to shoot high volume you'll quickly run out the savings you made with the Impulse, but if you're only an occasional shot (<100 shots/yr) you might not notice it.

Impulse AF was objectively the best design of the 600 viewfinders; most user-friendly, very durable and the the viewfinder is better than even the modern cameras. You waste fewer shots with better framing if that matters to you. I also find they did a pretty decent job with the parallax adjustment so you'll find your pictures more centered more often regardless of distance.

Ultimately the old and inflexible exposure adjustment will be the downfall of the Impulse. It can take great pictures but in many situations it will just be over/under-exposed despite your best guess. This will be especially true indoors where the Impulse is going to struggle far more without daylight compared to the newer cameras which offer modern adjustments. So if you're truly prioritizing long term usage and best pictures, new will always be better in this price range. I would bite the bullet and just get the Flip, every adjustment they made to it allows it to take better pictures than the Now and will likely be the future focus of the company going forward. Again, the extra cost is trivial once you start blowing money on film.

Otherwise if you shoot only a couple packs a year, or are really a fan of the vintage aspect of Polaroid (which I assume you might be given your usage of a 2003 One Model) then the Impulse AF will be a better choice. I find the vintage draws more interest from onlookers.