Why 3.5e?

Why not 5e? The breadth of character customization options - even if in the core and near-core books - means that even in games with a high turnover rate, every character feels unique. The advanced rules for wilderness and dungeon exploration, relatively high lethality, and magic item system complement the very exploration and risk-and-reward nature of the West-Marches game.

Why not Pathfinder? Frankly, I've never DMed it before, while I'm very familiar with 3.5e. There are a few things - magic items costing XP, level loss from raise dead, etc. - that are in 3.5e but not PF that I believe fit better into the free-form West Marches system (spending XP on a magic item isn't a death sentence if you aren't permanently behind in XP compared to the other characters). The general aesthetic of 3.5e also captures a certain retro feel that I'm going for in a way Pathfinder does not.

3.5e was the edition the West Marches was designed for.

How many DMs are you planning on having, and what will be max party size?

Our ideal DM team will scale with server size; right now, I'd like to get 2 more. Co-DMs are hard to come by, though, so at the moment it's just me. I wasn't planning on setting a maximum party size; if it become necessary during play, we'll cross the bridge when we come to it.

Which resources will be used by players, and how will they be distributed?

I have a list of approved books (By my terminology, the "Basic Resources" for the game; the first four Complete books + Core and EPH, basically). Anything from those books is allowed (minus a few explicitly mentioned tweaks and bans), and in the 5e AL style, if a player really wants to use options from an additional book for their character, they can pick options from one other book. This cuts down on option paralysis and powergaming to a big extent.

The d20SRD has everything a player needs to create a character in 3.5e using PHB+DMG. I have personal pdf copies of several books which I physically own that I'm willing to share with players via private message; from what I understand, sharing personal pdfs privately isn't even illegal. I will also, of course, tell my players of a certain book-aggregation website (first letter of "T"), so they know to stay away from it.

Just out of curiosity: how will you deal with the low-key imperialism of West Marches D&D? The subgenre, at least in my understanding, involves a lot of going out into the “untamed wilderness” and killing the “uncivilized savages (orcs, kobolds, etc)”, then taking all their stuff.

The West-Marches style was chosen because it is much easier for comrades with irregular schedules. All I can really say is that I am 100% aware of the imperialist (and at time racist) tropes of the West Marches style, and plan to subvert it in as many ways as I can, including:

(1). The setting itself is very non-western-european. You're more likely to be a scimitar-swinging dervish than a crusader-knight bringing Jesus to the unbelievers.

(2). I tend to play evil races fairly straight, against the more recent Eberron and 5e trend of having them just be misunderstood. Rather than compare them to real-world ethnicities (which is extremely gross; even calling a group of Orcs a "Horde" makes me feel icky), I tend to portray them as right-wing political ideologies given flesh. They aren't primitive tribal peoples just living their lives nor ravening hordes; Orcs are a people just as technologically and sociologically advanced as any other people, but they're misogynist, militaristic, theocratic, and xenophobic; they're fascists, and that's what makes them monsters.

(3). There are few humans in the Ohm Basin. Those human cultures which do live in and around there I hope are realistic and charitable representations of tribal people; and, to stress this, are not "monsters" to be conquered and slain, but people that I would reasonably expect neutral and good-aligned characters to respect. If there comes a time where the PCs are urged by a shadowy patron to push a human tribe off of their land so it can be settled, I fully intend that to be a moral dilemma and not an expected part of "just playing the game".

Is the gameplay going to be leftist (as best it can, given the genre assumptions and game design), or is this merely meant to be a leftist-friendly space?

Only a leftist-friendly space. Obviously, a lot of ideology leaks into the work, both on my end and that of the players. But the campaign has a late-Medieval, Renaissance-era theme that is unsuitable for outright Socialist (for the most part! There's always exceptions). In my last game, for instance, players broke into a museum to re-distribute stolen Dwarven cultural artifacts (mirroring similar thefts by the colonial powers in Egypt and Colombia), and fought a cult of bougie xenophobes who were sacrificing immigrants to make the crops grow.