-Just saw this thread, too late to matter but might help others.
-First off, a nice looking unit. For my needs, I like the lightweight, movable, and fast approach, not the big massive approach.
-Pump above or below is mainly based on pressure drop to get fluid to the pump inlet. With tight inlet line that doesn't leak air, in warm oil, and large lines, the tank can be feet from the pump. In most cases, It should be as close as possible, and below tank so the head pressure tends to push the fluid into pump. When it have to lift up, there is a negative pressure from that lift. Mainly, get as large as possible on the suction strainer, fittings, and hose.
-Tank baffle: Oh I do hope the baffle has lots of holes in it. Someone earlier implied the oil should go up and over the baffle like a waterfall. That adds lots of air and increases the chances of either aeration or cavitation. You want the oil to come back from return UNDER the fluid level, ideally slow down to a uniform smooth flow in a diffuser, then move as slowly as possible around the full tank distance. The point is to have as much time as possible for air to combine and move upward, while larger particles or water moves downward. The baffle is to direct the flow and keep it from short circuiting right from return to suction. Cut a couple lower diagonal corners off the baffle to direct the flow along the outer walls for cooling and to avoid any dead fluid spots. You DON'T want the baffles solid. Also remember cold fluid is really stiff, so the holes and space need to be quite large.
-Filters vs. strainers: Mostly semantics, is it a coarse filter or fine strainer ! Things with metal screens, 50-100microns or bigger, usually called strainers. Things with glass fiber or paper pleated elements are called filters.
-Filter 'ratings' are somewhat vendor smoke and mirrors. I won't go into Beta ratings for efficiency, Just get a good one for return flow. The 'rated gpm' is of course at some pressure drop, which depends on oil temperature and viscosity. If the vendor chooses to rate it with 10 oil at 160F temperature, for return line use it could go to maybe 30 psi drop across the filter, it can be called a '30 gpm filter'. To use on a suction line where the pressure drop should not be more and a couple psi with cold oil, it might be more accurately a 2 gpm filter.
-The bypass on a return filter is maybe 25 to 40 psi. Bypass setting on a suction filter is set to maybe 3 psi. Sometimes the spring & valve have two positions, sometimes they are different heads.
-Most of the standard oil filter sized return filters (with 1 inch fine threaded post and 1/2 or 3/4 NPT ports) I would use for about 15 gpm max on return line. Remember, the flow out return is greater than pump flow by the ratio of cylinder areas: Closed side area / rod side area might increase the return flow by 30 to 50% over pump flow.
-I would NEVER use a suction filter for logsplitter. I would probably use a suction strainer but vastly oversize it. What Northern calls strainer for15 or 20 gpm, I would only use for half that flow or less. They are not accounting for cold temperatures.
-Strainer or not is actually a difficult decision on case by case basis. It is to prevent weld berries or stray bits of trash from damaging pump, but can of course cause cavitation. I usually use a very large suction strainer (1/8 inch holes, or 60 mesh at the finest is using a screen) with most industrial systems. There, we fill the tank through a quick coupler to force oil through the return filter. For splitters, given that someone might pour in oil from a jug in the back of the truck with bits of dirt, wood chips, and maybe the occasional nut or bolt worked loose from inside the tank, I would use a large strainer.
-Cavitation is not the same as aeration. Yes, air can come out of suspension when there is a cracked inlet or loose hose. Cavitation is when the fluid pressure drops low enough in the inlet to pump that the oil actually 'boils' or vaporizes some volatile portions into a vapor. At a slightly different spot, the bubble collapses and the liquid around the vapor bubble slams into other liquid and the shock wave causes impacts to the metal parts of teeth, etc. Somewhat like detonation in a piston engine if you understand that situation. Anyway, cavitation is very hard on pump and erodes the metal surfaces.
-For cold start, heating the tank is a great idea. Remember to heat the pump and inlet line because the hose is still thick fluid. The magnet stick on heaters work very well on the tank at the strainer location.
-Cleaning the tank: Sandblasting the parts is good, blasting the finished tank is not. Bits of sand get trapped in crevices and weld joints, Sand, weld slag, or the mill scale on plate surface are essentially sharp particles of glass and very abrasive. They get released later when things flex or bounce. Sanding the mill scale off is good, or using pickled and oiled steel (not common in the scrap yards), or using stainless steel if you have it, especially for the lid. Phosphoric acid degreaser from the Napa store, used for degreasing body parts before painting, does a good job of cleaning. I would never paint the inside of tank. There are some specialized epoxy coatings, but mixed results and not for amateur application. Instead of rust particles, you can get paint flecks or big chips coming off.
-The splitter valve might be reversible in direction. Sometimes the fixed point on end of the lever can be mounted above the spool, or below the spool. That reverses the direction of spool motion. I don't know about the Prince valve: Is there a little chain link type connector there with two mounting bosses where it can be installed?
-Time to go to work. At some point I'll get back and add some more hydraulic info or formulas.
kcj
Great size for someone who doesn't have a lot of space to store it and yet big enough to get the job done. 
