The biggest "plus" is having an A&P to do the work yourself....
If you had to pay labor to convert a 150 to the bigger engine, that is when you would probably be better off buying a plane already converted. Finding a 150/150 with a run-out engine would most likely sell for quite a bit less than one with a mid-time or newer engine, and that could be a great course to take-then you would only have to deal with a major o/h rather than a major conversion.
