I'm thinking in buying this game but I have two questions:
1. Can I install mods in the GOG version?
2. Do I need the expansions to use mods?

Thanks and I hope you can answer me

1. Yes
2. Not unless they are specifically tied to those mods.

All mods from the nexus and those from the forum of Taleworlds (dev) work with the gog version. Most are Warband related and there is no need then for the 2 dlc's. Read the description of the mod you use carefully.
I'm using some mods myself with the gog version. I've download them mostly from nexusmods. I use a mod called female faces a mod called face 1.3 (adds hairs) and the more women mod and i use tweakmb. Works well with the latest gog version.

There's a whole bunch of mod info over in the Mount and Blade subforum. There you can confirm what the folks above have said. https://www.gog.com/forum/mount_blade_series#1517560116

Regular M&B:W is great already, and mods just make it that much better. Myself, I thoroughly enjoyed the Prophecy of Pendor conversion, though I recommend getting some vanilla time under your belt before jumping into that challenging mod.