Rendering the scene in unreal (or any real-time engine) use GPU.
Building the scene lighting (lightmass is a G.I solver) use cpu+ram. You already have a good setup in that regard.
The 750ti won’t get your far. Look for a gtx 1060 if budget is tight, otherwise 1070 or 1080 with as much vram as possible on it. Welcome aboard!
When you use displacement, planar reflections, high resolution, supersampling and you want to record in high fps on top of that? the 750 will explode lol!
The gtx 1060 is an incredible bang for your bucks. I think the performance are similar to my gtx 980 that I bought 800$ last year
The scene above is kinda high quality and the baking could take hours on a single i7. A gtx 750ti could potentially run it decently but definitely not in more than 1080p and forget recording footage smoothly with that gpu.