Hey Miroslav, sorry for the late reply.
Well, here'swhat I do. I'm not an architect and i'm also fairly new to Architectural Viz. I have this app on my ipad that i boughht for about 2.99 called 'modern houseplans'. There are many though... There's one for spanish house plans, beachfronts, gerogian etc. from there, I model the plans just as I would if I was approached by an architect. From there it's on to the render.
I suppose the people who upload ready made models expect persons to use them so I don't think there would be much of a problem but maybe you can make a footnote stating the person(s) who created the model.
Hope I was of help.
Cheers,
Valdano