You shouldn't include modloader, the same way you don't include img tools, you ask them to get a img tool, is that right?
The same thing, you can just ask the person to get modloader, and what's the instruction to install it on modloader? "Just drop this folder at modloader folder, it's done", you don't even need to do any kind of script, modloader works with anything made since the first days of modding.
You can give two options on the readme, either modloader or a img tool. Come on, that won't hurt and will help modloader get more adoption.
Also, Steam version (and 2.0) do not allow gta3.img modifications, the game will hang, there's plugins out there to solve this issue.
As BLITZ mentioned, installing thought modloader would work since it doesn't touch the game files but as James mentioned, Mod Loader do not support Steam executable yet, but IT WILL, as you can see on our goals.
few as possible third-party things, because always better if game works only with their native functions (where it is possible).
Not really, the game breaks if you use badly coded plugins (or scripts). I'm no newbie, my coding skills ain't broken.
In fact, ModLoader even fix some game issues, such as the "sound bug".
And to be honest, do you really think that people that is going to use HRT is going to have a unmodded game? Sure they will