There are some additional rules that will be established during the live stream for the build rolls and decisions tonight concerning mercs. Basically these are pinned in a channel right now:
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you can trade with yourselves here in this channel, unless we find it becomes too much to handle and will create a new trading channel for this event only. the idea is to keep trading within the event so you can't just twink shit from outside players (despite it being a chuck challenge full of twinking, chuck only trades with other SP players to help him gear his 50 characters).
As far as mercs are allowed, I will add this to the post as well. Chuck does mention in the beginning of his guide to get a merc opposite of your char (ranged char should get melee merc, etc), but doesn't list any gear for the merc. In this case, unless the guide says explicitly to get a specific merc (one of the assassin guides says to get an a2 then a5 merc), mercs will not be allowed until you begin your heavy farming. He cannot join in to help you get charms or clear bosses unless a merc is mentioned in that miniguide.
For in-between gear, the idea is to stick to what the miniguide says. This doesn't mean you need to farm Andariel for all of your TU's at lvl 20, but the progression should be as follows:
Try to have at least all of your TUs and RW's by Level Challenge 1 (don't need to all be uptiered, but at least have them).
Mid-game gear listings should be acquired (some optimistically depending on trading/rng) by the time you have about 5-10 charms and/or begin your farming stage.
End-game gear should be acquired when you have nearly all charms done or all done and sigged and farming is extensively over (i.e. try to have end game gear by the time of ladder reset for the big pvp event -- exceptions include xis rw's because they can be draining to farm, but try to get those as well as possible).
Don't use any uniques/sets/runewords/crafts that aren't listed as per his standards for this challenge., because... it is a challenge afterall. You can use some rares or magics as you pick them up to see if they have your proper oskills and such, but don't create your own gear sets just to make the build work.
Do your best to finish the characters. Suchbalance said it might be nice to have a big pvp event and create some type of boss/aoe farming showcase with the characters, almost like the 2017 trailer, but more endgame.