To answer one of your questions....the bell housing should be dry. As far as fixing the pins, it is cast and not easy to weld correctly.....but I think if you find the right guy, they can be fixed. Worst case scenario is you get them welded back on to keep the bell housing dry and reasonably strong structurally and either leave the loader off or fabricate different mounts..... which I would be working on a better mounting system anyway.
If it was running good when parked and it didn't get water in the engine, I bet fresh fuel and filter, oil and a fresh battery will have you up and running. NO way I'd take $350 for it, that's close to the price I had to pay for a used hydraulic pump for mine.....lots of good parts there if nothing else. Here is a link on how to bleed the 3AE1 engine
http://www.tractorbynet.com/forums/files/iseki/327463d1373828160-tl-1900-a-tk8q-jpg One thing to watch for....since it's been setting so long, if the owner didn't lock the clutch pedal down, the pressure plate may be stuck to the flywheel......cross that road if/when you get there.
The electrical switches on the fender are for your 3 point hitch. The toggle switch is for the hydraulic tilt ( nice feature) the two knob switches are probably used with sensors that attached to original Iseki implements....most likely mainly for a rice paddy tiller. If you have a separate 3 point control lever to raise/ lower the 3PH, you may have to adjust/play with the knobs to get them in the right setting. If you don't have a separate control lever, they are the main controls for the 3PH raise/ lower and fall speed control.
You definitely have a TL model. It is either a 1900, 2100, 2300 or 2500. The first two digits indicate the amount of PTO HP, not engine HP ( engine will have 2-4 more HP than the PTO rating). It is probably either going to have a Isuzu 3AE1 or 3AF1 engine. The TL1900 used the F model (1170 CC) and pretty sure the other models all used a 3AE1 ( 1296CC). I would look for 3AE1 or 3Af1 or maybe 1.17L/1.2L or 1.29L/ 1.3L cast somewhere on the engine block.