Don't worry to much, I would be looking at your oil pressure when warm not lower than 15 and preferably above 20. If it's at say 12 get it checked out, I think Nissans safe min is about 14.5 but don't quote me on that (Cue the techs).
Also a rough idle may be a sign, when getting the car serviced ask for a proper pressure check with a workshop gauge.
The oil won't leak outside the car so your unlikely to see any oil on the floor.
Replacement Inc labour shouldn't cost more than 850 at one of the specialists on this forum and most do it under 800.
The beauty of the 350 over the 370 in this case is the access to the front of the engine doesn't involve removing the front end of the car as it does on the 70 so labour cost isnt as much.