Outside of looking for pre-owned or possibly a refurb/return from dell or lenovo outlet store your budget is a little low.
For lightroom use you are going to need a core i7 HQ serries, the HQ series is 4 core 8 thread but the U series (what will be in the lower priced laptops and ultrabooks) is only 2 core 4 thread. You will also want at least a GTX 1050 GPU in it as lightroom can utilze the CUDA cores in NVIDIA gpus to help with processing/rendering.
New you are looking at more like $800-900 for this setup.
And FYI any AMD A8/A10 laptop will be absolutely awful for this, worse then an i3.