Do I need PCIe 4.0 or PCIe 5.0?
Generally, no. PCIe 5.0 or 4.0 is great if you have a GPU cluster. It is okay if you have an 8x GPU machine, but otherwise, it does not yield many benefits. It allows better parallelization and a bit faster data transfer. Data transfers are not a bottleneck in any application. In computer vision, in the data transfer pipeline, the data storage can be a bottleneck, but not the PCIe transfer from CPU to GPU. So there is no real reason to get a PCIe 5.0 or 4.0 setup for most people. The benefits will be maybe 1-7% better parallelization in a 4 GPU setup.
Do I need 8x/16x PCIe lanes?
Same as with PCIe 4.0 — generally, no.
PCIe lanes are needed for parallelization and fast data transfers, which are seldom a bottleneck. Operating GPUs on 4x lanes is fine, especially if you only have 2 GPUs. For a 4 GPU setup, I would prefer 8x lanes per GPU, but running them at 4x lanes will probably only decrease performance by around 5-10% if you parallelize across all 4 GPUs.
What is NVLink, and is it useful?
Generally, NVLink is not useful. NVLink is a high speed interconnect between GPUs. It is useful if you have a GPU cluster with +128 GPUs. Otherwise, it yields almost no benefits over standard PCIe transfers.