Agree -- bus service throughout Portugal is very good and comfortable; I've used both the trains and buses extensively throughout Portugal and truly appreciate both. My only complaint about the bus stations is that I've found it difficult to figure out which bay will be used for the departing bus (I'm admittedly Type A... The buses have destinations and bus numbers on the front, but if you don't know the geography and the routing, it can be difficult. I've seen two buses arrive with the same route number markings but going to different locations). I haven't been able to figure this out yet (I rarely stand in line to buy tickets, I buy them online instead. But perhaps buying them in line would provide that bit of info). Please note that the buses usually arrive just minutes before the departure time, so try to not let its absence stress you. Fortunately, the drivers pay close attention to each person getting onto the bus and direct you away if you make a mistake.
The Porto main train station is Campanha. It's not central, but it's an easy transfer to the line taking you to the VERY central Sao Bento station, about a 5 minute ride away.
There are several places to catch buses around Porto, as I recall (there are 5-6 different bus companies, often operating out of various locations). I'm not recalling one distinct bus station... perhaps I can get some assistance from others more knowing on this topic. The bus and train stations are now collocated in SdC, an easy walk outside the Old Town area.
I used google maps a lot, in addition to Rome2Rio, to figure out my transportation options around Porto. There are many options, generally quite easy to use.
Locally around Porto, Uber is also very inexpensive to use, if needed. I'm guessing cabs are reasonable for short lifts, too.
Good luck!