Do you have enough radiants to heat up the grim repeater if you were to craft it now? If not, that is one more reason to hold off until winterfest and then decide. If you do have the radiants, I think I would go for the Grim Repeater right away because in the end they are just 2 skins that are equally unique. You can craft the peppermint repeater year round as long as you collect the materials during the event, so you could always get it later. That said I personally like the normal Grim Repeater skin more than the peppermint because it fits with more outfits, although the red bullets on the peppermint are cool.

Yes the mission is not that easy if you aren't prepared loadout wise. You will want 5 star gear with damage bonus for your weapons, the defense type isn't as important because there are enemies that deal elemental, shadow and piercing in the mission. Getting the DLC is great because dark retribution and warmaster rocket hammer are both very powerful (as long as you don't forget to heat them to lvl 10). That said, Acheron/Obsidian edge, Grim Repeater, Blitz Needle, Vortex bombs, Gran Faust, etc. work too.