Early 70's is correct. Without the model number it can't be pinpointed any closer than that. They had red stripes for a number of years.
That era Mercs could be tricky. They have multiple carbs and if not perfectly adjusted they would grenade the top cylinder. If the motor is low hours, the price is right. To be made reliable it woul;d need a real good Merc mechanic to recondition it. That includs, carbs, water pump, tune-up etc.
I have a 1972 200 (20 hp) and it is tricky, and they were known to change stuff around from year to year without apparent reason. Watch for frayed wires going to the stator; they may need to be replaced. Good price though if they run well and will probably be fresh water motors up there.
Yes it would be fresh water. I will go look soon. Thats is the only motor ive found around here thats 35 to 55hp i need it to go on my starcraft for fishing. I dont know much about motors though. just boats and fishing