@Peter Carboni I have lived in, and right outside of, the city for many years, but I never invested there. The reason I didn't invest there was because we wanted to invest in Maine where we will be retiring, but I have some insight into the city. As you are house hacking you will want to be real careful which parts of the city you choose to buy your property. I'm not sure how familiar you are with the different areas, but there are many high crime neighborhoods. A lot of these questions can be answered by your Reatlor/Agent.
After you find a potential investment, you would need to have a home inspection to identify problems with the home. I would never skip the inspection.
Act professionally, but don't worry too much about what you wear as long as you are presentable. The area isn't a high net worth market where they need to see you in designer clothes before you are taken as a serious buyer.
As far as which specific streets or areas to avoid? There are so many, but there are also nicer areas too. The far North End is generally safer than most of the central part of the city. The South End, as you mentioned, has nicer areas down by the beaches, but it also has bad areas a short distance away. Though all this is subjective so I can't say for sure what you are looking for in a neighborhood.
Nearly all of the triplexes are old, but many have been updated over time. This is where an inspector can tell you what is to code and what will need to be updated.
As I haven't invested in MA, I can't speak to the tenant laws. The only thing I would say is that there are never many multi-family homes for sale in the city, compared to how many there are in total. The rental market has been strong for a long time and landlords aren't trying to sell. This tells me that tenant laws aren't as restrictive as other inventors might lead you to believe, but please confirm this with your agent. Hopefully your agent is an investor as well. If not, you may want to consider finding one.
If you have any other city specific questions, please reach out.