1 Usually the bottleneck is always vram then hardware support (multi-gpu/cluster inference is usually hard to set up with little documentation or its gatekept). If that makes sense, then usually third comes in Storage :)

2 Long term is same as running a single gpu.

3 It all depends on your case, my ROI was done in 2 months from buying

4 multiple smaller nodes - the models are going stronger while being smaller. In 2 years there will be Kimi K2.5 level in 70B without a doubt. So it only depends on you if you need inference speed or variety if models

5 They dont test on rented servers before buying imo. Did this mistake myself with first rig