i personally prefer sculpting, as it allows you to focus purely on shape and worry about topology later. also, it's normal to do at least some sculpting as it allows you to make bump maps for your model. I prefer the multiresolution method to the dynamic topology.
I think that blender will be fine for texture painting, it has a lot of nice tools for it. here's a tip for it that I didn't know until recently. on your brush settings, there's two settings called spacing and jitter. spacing makes the brush add color in a dotted way(you'll understand what it does if you try it out) and jitter makes it more random. this can be used to make your textures look a lot less uniform, and it looks great! this can also be used for sculpting.
you should aim for low poly, but don't get too low! game engines can handle more than they could ten years ago, and having extreme low poly can lower the game graphics quality a ton.
If you're trying to make characters, a good front and side reference image is a must!
In order to optimize, just put the high level detail in a material versus the mesh.
one more useful tool if you want to sculpt. the mask tool is pretty obvious, but the amount of uses for it is very high. also, if you want to draw the mask quickly, draw out the area you don't want to be masked, and press ctrl+i to flip the mask.
I hope this is helpful, and please comment if you'd like more info.