If you absolutely mustn't spend any money, you could try out Blender's Cycles engine. It's not a realtime renderer like you would see in a game engine (although it does run on an Nvida graphics card if you set it up) and it doesn't follow most of the same conventions as e.g. UE4, but it is physically-based and it is free.
I tossed together a Cycles shader that imitates Unreal Engine 4 materials. So if you use Blender (or if you at least grok setting up materials in it), you could give that a try.
You can use grit game engine for it http://gritengine.com you can talk to people on the forum and chat. Spark is the programmer you can ask him anything about it.
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Cycles shader
It can be good for visualizing the texture work, though it won't be the best thing I think if you want final renders for a portfolio.
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Don't think 3do is free unless you actually bought dDo or nDo2.
Haven't tried it but it's free... Just read about it today, it's called PBRViewer.