Regarding the fan issue, you don't need them when dealing with a liquid like this. They use aquarium pumps I think to move the oil for cooling already, just set up the return to point at the CPU and Video heat sinks, that will work just fine. I would not mount the power supply inside the tank though. It would be much easier to deal with the drives and external power requirements with a dry supply IMO. The fewer cables that need to snake out of this monster the better.
One problem I have seen with using mineral oil and other viscus oil like liquids in other aplications. Depending on the environment outside of the tank you can encounter a wicking effect were the oil crawls up the cables. Even inside the cables given enough time. It is not as fast or disruptive as water in this process, but it can impact other hardware and make a mess over time.
Also you will need to do something to cover or mitigate the smell of it, LOL. it may be fine in the garage, but try living with this in your house LOL, major wife/girlfriend agro.