I think a Pacific 550 is closer to 18ft. Today you'd expect an 18ft boat to be rated to around 115hp, but as a 70's boat I would guess it was rated to a max of 60-70hp as it was competing with the likes of Shetland's 535 which was rated to 65hp.
As to the type, depends on your budget. If you're looking for something older and cheaper there are makes to avoid such as Chrysler and Force, but stick to makes that are still around and you should be OK. An older engine is likely to be a carb 2 stroke - nothing wrong with them, especially the last of such types such as the Mercury/Mariner 60, Tohatsu 60/70, Yamahas etc but they will use more fuel and make more noise than the latest generation of engines. Take a look at the out board buying guide in this section of the forum for excellent advice about what to look for.
Move your budget up to mid range and you're into old tech 4 strokes - some are very heavy (e.g. Mercury/Mariner 75) compared to what the manufacturer would originally have had in mind for a 70hp rating, or newer old tech 2 strokes such as the last of the Tohatsus, Mercury/Mariners, Yamahas etc mentioned above. In my experience a carb 4 stroke doesn't offer a huge amount over the last of the carb 2 strokes, but some such as Honda are quieter and smoother than others - but Honda is another make where the older model 75hp is a huge monster of a thing.
Move your budget up again and you're into the newer electronic fuel injected two strokes and four strokes - now you start to see advantages in easy starting, low fuel use etc. If you're in this market, you'll be buying a make that is still currently available and there's not a lot to choose between different makes, apart from the weight - e.g. Mercury Mariner 80 very heavy, Mercury Mariner 60 very light, Evinrude very light - so apart from weight considerations go for a good deal on whatever turns up or whatever make is well supported by a good local dealer.