Sorry nobody has replied here, they're probably answering questions like "how do i mod ships" that come up every five minutes.

#1 and #2: I don't know about any hard limitations but the Kol Battleship is around 2-3000 iirc. The engine handles high poly very well. We currently have a ship that's about 4-5k running around with a 2048x2048 diffuse/normal/bump and even in playtests with 30+ of them the game runs fine.

#3: What a coincidence, as I've been trying to get some working for two days now. Instead of the obvious, simple way of simply creating a dummy in 3DS, then pointing the weapon in the entity file to that name, the game actually runs through your dummies and arbitrarily assigns them to weapon banks, often hilariously failing and shooting out of the origin of the model. Weapons are named Weapon-0 through Weapon-whatever, Exhaust is named Exhaust, Ability points are named Ability, Hangars are named Hangar. More than one point can be created under the same name to spawn duplicate effects.

#4: Yes. I have a few I'll probably release soon, but you should look at http://janvanderweg.com/SOASE/index.html at least. The mesh import/export tool now is sort of a piece of shit and crashes, but I'd gladly host the old 1.1 version if you use 3DS and need it. Entity file stuff is not really something you can learn through a tutorial, you just have to do it.

#5. I guess so, it's not that large but neither is the modding community.

Sins is not really that mod friendly - the buff system is extremely powerful but nothing else really is. If you enjoy the modernity of the engine along with its base gameplay, it's not a bad place to start a mod, but prepare to run into hundreds of hard-coded limits and a lot of frustration.