This ideas seems interesting, and i have thought similar myself, now have the water pumped in a septate container is not the most efficient way of cooling the oil, you would be much better pumping the oil into a radiator outside the tank, and yes the pumps could handle it, as Mineral oil, is in fact less viscous that water, and less dense, which is why in floats on top of water and water sinks below it, now on the subject of condensation as mentioned, i doubt this system would be able to chill the oil cold enough for condensation, as i presume the water/antifreeze solution won't be to much below freezing, and be able compete with the immense heat of gpu's and cpu's, so a more late autumn.. not mid winter cold, but i do like your idea.. a lot. And it doesn't matter if the water is below freezing it is what's in contact with the air that will condense the water moisture in the air, i.e. the surface oil in the tank, lets say it did get the oil that cold, its possible, now simple solution, air tight tank, not as hard as you may think, but you will still get a little, from air trapped inside, but minimise that by filling to the brim, sounds simple right?, but you will never 100% get rid of any chance of air moisture condensing on the surface, and going into you tank still, but as i mentioned before, water is more dense than mineral oil! so it would there for sink to the bottom, now to me it was still a problem as i planned to circulate my mineral oil, and the water would all get churned up there, and eventually inevitably short something, but no matter for you your system doesn't include this circulation of the oil. So things to keep in mind of you system ;
1.DON'T slosh the tank around, if cold enough to create that condensation build up at bottom of tank, you don't want to churn it up into the oil, it may be such a small amount because of the precaution i mentioned (such as airtight, fill to brim, not cold enough when against cpu/gpu) you may not be able to see the layer!
2. Maybe incorporate a plug at the bottom of tank, or bottom at the side, low as possible, underneath would be great, really tiny hole, so from time to time, if you cooling systems works better than expected, and you airtight container not as expected, then you can drain out the visible water.
3.Have everything slightly elevated from the bottom! Simple, that way if water does form at bottom there is no problem given no fans are arranged to create a current moving water around, also don't worry about convection currents, the mineral oil would have to become very very cold, and the water very very hot, for the oil to sink below it, and water to rise, its near impossible in this theoretically closed system.
I at first missed the part about the cooling system going to some water blocks first, this is a good idea, and will make it much more efficient and drawing away heat, have you heard of phase change cooling, very expensive, but its going to be a lot better at cooling, than your a/c and can be used in the same way as your design.
You asked if anyone had any of their own input's or ideas, I've given a few, but here's three others...
1.Bigger radiator in the tank if you have room
2.Radiator up highish if possible,as the oil becomes hot it rises, so the hottest oil, in need of cooling is up by the top. But the top of it needs be no higher than the top of the mobo, as this is the hottest area of oil actually in NEED of cooling, have a fair volume of mineral oil above, to little and your going to create a freezing surface, not great for condensation build up, with a lot of volume above you have an area for the hottest oil to collect as well, making it cooler below by your mobo. The particles at the top will eventually loose their kinetic energy and fall back down, in a convection current(independent of any possible water below, as density difference is to great)
3.Bigger radiator means need of a bigger pump
4.Maybe need to base visual design around mechanical design of it, stuff mentioned now and earlier
5.Shorter the tubing the higher the cooling efficiency,
6. But deep sea fishez in the water compartment... (radiator), kidding... fish are expensive..
Oh and I still think it might be worth you looking at phase change cooling, extreme cold!
And more!, not sure if you have invested in a hard drive, but an SSD but definitely be worth the extra money, you can compare SSD's and HDD's in numerous ways but in this case, its got to be the fact that the SSD could be submerged into the tank, the HDD would be damaged if put in and the annoyingly kept on the roof of tank or something, if creating air tight tank, allow for lots of drilling ect in the acrylic box for the wires of any optical drives, you mouse your keyboard, monitors, Ethernet ect, and of course, the on button, and various other switches.
If mobo is already purchases no biggy, but try get one with ALL solid capacitors, not just solid capacitors around the cpu socket, and to attach the water blocks to the cpu before the radiator, as i think you are doing, do NOT use thermal paste, the mineral oil will degrade it, use 'Indigo extreme'
In terms of pump size, the temperature will not really affect it, you will be able to find out from the manufacturer the size needed depending on the size of your radiator and how fast you want it to circulate. Keep in mind phase change cooling though.
Hope this helps, any questions I'l be happy to help some more, I just did a post on some idea's of my own, feel free to take a look http://www.overclock.net/t/1332180/mineral-oil-submerged-phase-change-cooling-project