I've had SIBs before and am looking for a new one. I have recently spent hours researching it in this forum and on the internet generally.
Everything has its pros and cons and everyone will either say "Get what I've got" or "Ooh, I had one of those, never again" - often talking about the same boat!
I have found that Zodiac's website is the best in terms of dimensions and specifications, power recommendations, etc. if you are looking for new.
If you are looking for second hand, decide what you want first, then look for it. A bargain is not a bargain if it's the wrong boat or engine for you.
A SIB of 3.4 metres or so with a 15 hp engine is a pretty capable boat and in good hands will cope with a lot - but when things go wrong at sea, it can be sudden and catastrophic. I remember playing in my boat in big waves off Fort Bovisand and a reflected wave crossed the incoming waves right next to me and it was like a depth charge going off. If I had been 2 metres further in that direction, the boat would have been over and four of us in serious trouble.
The same with a long easy coastal journey going wrong suddenly when you try to beach the boat in surf.
I strongly recommend at least going on something like an RYA Powerboat course. You will learn a lot. Anyone can drive a boat reasonably fast in a reasonably straight line and have fun. Slow speed manoeuvring, man overboard drills, picking up mooring buoys, and so on are a bit less obvious.