5e Adult White Dragon

[Lord Il Palazzo]

The party will have some Magic Items. The Paladin might have a nice Longsword, The party will have random Wondrous Items, and probably will all have Cold Resistance at that point.

I personally wouldn't give the whole party cold resistance unless they knew the fight was coming and went out of their way to prepare and find ways to resist cold and even then I probably would make it pretty challenging to get resistance for everyone (maybe one or two magic items and then force them to take spells or come up with other plans/methods of protecting themselves). Handing all the players resistance to one of the big boss's most potent offenses just doesn't sit well with me. It removes (or at least significantly hampers) a threat that the DM (and the dragon, in-character) should be able to rely on to make the fight challenging and it keeps your players from having to work to mitigate/neutralize that threat (including by the tactics they use, spells the prepare and by planning and seeking out defenses beforehand) which takes some of the coolness out of giving them a "We beat a dragon!" story. Maybe it's just me...

Edit: Another thought is to make at least some of their cold resistance concentration based, like a ring that allows them to cast Protection from Energy X times per day. Having to decide who gets that protection and to work to keep the concentration up (and decide if it's worth sacrificing another concentration spell they could be using) would add a more entertaining element to the fight than just walking in knowing the dragon's breath will only deal half damage.