What is the best software to choose (taking in account a not so large budget) for building 3d levels with portals, lightmaps and some sort of partitioning like bsp?

On a limited budget, you could simply use the tools that come with popular commercial games (i.e. UnrealEd).
From an artist's point of view, using his/her favourite modelling package works as well. You can export the level to whatever format your game or its tools accept and do lightmap computation and BSP creation as an offline pre-processing step.
Ogre3D offers a number of tools for that and I'm positive that other free rendering/game engines have something similar as well.

Good luck!
Pat

I'd recommend using Quake (if you want something free), but the best software out there for this is Cartography Shop ($65?), well, in my opinion anyway.

Thank you very much for your input, I really appreciate it :)

Found this link for the Unreal2Runtime (including UnrealEd), haven't had the time to check it out, but I'm posting the link if anyone who reads this thread is interested: http://udn.epicgames.com/Two/UnrealEngine2Runtime22262002