1) How much tris can I use for characters?
Just for example Mario from Super Mario 64 has 752 tris.
2) How much texture resolution should I use for characters and objects?
It really depends which platform you are trying to model your game after. N64 games for instance will have more polys and lower resolution textures, while PS1 games will have fewer polys and higher resolution textures. If you want you can hop on our Discord and let's have a chat about it! https://discord.gg/KYrxEXe