Which version of the 836? Some have 80+ Platinum PSUs. But yes, you will be limited to using Super Micro power supplies only.
As for you more general question, if you want to get the most efficiency possible, you have to match the PSU to your power requirements. Which is difficult unless you can test the exact efficiency curve of each PSU you are looking at. I made a quick graph of some of SPCR's PSU reviews which you can see in my PSU Efficiency Comparison
. I just picked out a few PSUs, primarily Seasonic and then also some high and lower wattage models. Where you see a difference between the higher wattage and lower wattage models in general is below 200W. Up to 100W, the low wattage PSUs have a good advantage over the high wattage ones. By 200W, things are pretty even. Above 200W, the higher wattage PSUs generally have the advantage. Of course the Cooler Master shows that the wattage itself doesn't automatically mean anything as it is below every PSU at every wattage.
But given your setup, you probably will never be below 100W. So really, the difference between a 300W and 1000W PSU won't be much. Either one would work fine. And while having headroom is good, I wouldn't expect a good quality 300W PSU running at 250W to have any more issues or a shorter lifespan than a 1000W PSU at the same load. And you won't have that choice as you'll be limited to the SuperMicro PSUs. I would go with the 80+ Platinum if you can. I would assume this thing will be running 24/7 as well. Note that these PSUs run at all times when plugged in. Even if the server is off. So they're not good for a system that will be sleeping or turned off often. Which you wouldn't expect in a server.