Well, not much good to tell you, but there are some things to check. To start off, if it is really running good, the maintenance issues below may just be the problem. Take you time and go through the following.
Are you certain water is getting into the oil from the coolant system? If it sat for a while, it may just be condensation. If the oil was not changed in along time, it could be a milky sludge buildup (again from condensation). Fill the radiator and check to see that it holds, and/or if there is oil in the radiator. If there is oil, and/or the water level in the radiator goes down, you've got a cracked head, blown head gasket, or cracked block.
Also, are you sure it's water in the oil? Could it be hydraulic fluid from the touch control unit? The answer is yes, it's a common problem. Check your touch control unit for fluid level.
Low oil pressure could be multiple things, not necessarily connected to the above. Again old oil or sludge in the system could cause low oil pressure. Often the oil pump pickup screen is plugged with sludge, especially if the tractor sat for a long time or did not have frequent oil changes. The oil filter could be plugged too. The gauge could be bad. Most tractors show low oil pressure at idle, which comes up at full rpm, that's normal.
Blow back could be from a cracked block, however, in these tractors it's commonly caused by stuck or worn piston rings. These tractors often sit for too long and/or are idled around all of the time causing ring problems. Many folks here (including myself) swear by the use of SeaFoam in the crank case and in the fuel for treatment of stuck / carboned up rings. However, this is kind of a last step if you've determined that the block and head are ok. If they're cracked, SeaFoam won't do much to help.
Notice a theme here? Some of what you have described could just be because of lack of maintenance by the previous owner. If it were me, I think I'd do the following:
1. fill the radiator and check for oil in the radiator. (if there's oil, you've likely got a cracked block or head or head gasket problem)
2. check for radiator leaks (make sure it's not just running out on the ground).
3. check the existing oil level
4. check & top off touch control fluid level
5. run a little to see if coolant level drops or if touch control level drops.
You should be able to tell from this drill if anything is getting into the crankcase that shouldn't be. If you've gotten this far and the coolant isn't getting in, and the hydraulic fluid isn't getting in, it's likely condensation buildup.
6. drop the oil pan and clean out sludge from pan and oil pickup screen.
7. remove and replace oil filter, treat new oil with SeaFoam (also remember to change the oil in the air filter)
8. run with SeaFoam to see if blowby settles down and oil pressure comes back.
I'm sure others will chime in but these are my thoughts.
Keep us posted and keep asking questions. You'll get through it.