Hi there

Personally I would get two GTX1070 and still if you have soare PCI_E slot then use GTX980 as well there, more GPUs you have faster render times will be...

I'm using or running multiple GPUs or 4GPUs setup with Asus RTX 2080Ti Strix, EVGA GTX 1080Ti,EVGA GTX1080 and Manli GTX1080 for rendering and I can only comment on my tests in Blender, my Asus RTX 2080Ti Strix will render Blender Classroom in 75 seconds with OptiX and 4GPUs(RTX 2080Ti Strix, GTX1080, GTX1080 Ti and GTX1080) will finish that in 42-45 seconds without the OptiX and Asus RTX 2080Ti Strix without the OptiX will render that in 2 minutes and 19 seconds that's in Blender Cycles

In E-Cycles same scene it will render in 1minute and 44 seconds that's on Asus RTX 2080Ti Strix and without the OptiX and same scene with 4*GPUs(Asus RTX 2080Ti Strix, GTX1080, GTX1080 and GTX1080Ti) will render in 32-36 seconds and just two GTX1080 will render that scene in E-Cycles in 1 minute and 29 seconds

Didn't test new Poser 11.3 update yet how RTX is suported and how fast it is against my other GPUs

If budget allows then yes RTX is way to go with multiple GPUs for sure

Regarding buying used GPUs, get EVGA which have transferable warranty which will only help if you have problems or issues down the line or get GPU which offers best warranty, if GPUs been used for mining, this won't make difference because mist of the people if they use them for mining they usually run them at lower power limits and most of GTX1070 have been used for mining ETH

Not sure if Poser will support OptiX if yes then happy days,but still would love to see E-Cycles or AMD ProRender, Corona Renderer in Poser rather but that's my view

E-Cycles its clearly faster than Cycles with OptiX, AMD ProRender its similarly fast as Cycles and Corona its fast as well

Hope this helps

Thanks, Jura