Superb advice above, I won't repeat what's in the sticky but you are pretty much looking to buy the same end result as the vast majority of the SIB users on here.
I'll just add a couple of points, someone posted on here earlier this week re' another which engine? question and summed it up very nicely and concisely roughly along the lines of:
"Buy the biggest engine you can manhandle from your car (and garage) to boat up to the maximum your SIB will allow".
It's as simple as that - if you cannot get it into/out of your car and up/down the launch area, remembering that for some this is a steep, stony long beach then it's no good. I'd like a 25 on my Aerotec but the simple truth is myself and my ship's crew cannot manhandle that weight any more up our beaches, yes, when I was 20 but at well over double that now forget it, certainly up and down shingle and pebbles. So, a 15 it is.
And secondly, your choice of SIB must be governed by where you will use it and the prevailing weather.
Again, here on the Sussex coast of the last say 10 the times we have been able to go boating, maybe 2 have been flat calm, the majority minimum F3/4 and last three times, random weekends mid summer as yesterday have been into F5.
Simple truth, having owned and run a typical Zodiac (Fastroller) 340 HP floor SIB for some years (+ the odd solid floor model) and now an Aerotec the Aerotec is streets ahead as a usable boat in the weather we typically experience. Simply owning the Aerotec with it's unique proper V hull all the way back to the transom makes boating safe and possible for us.
Yesterday, amongst the kite surfers and wind surfers there were no other SIBs to be seen, I've been caught out in a flat bottom SIB when it really blew up and safe and comfortable it was not. The Aerotec is unique in that it allows you to cope with bigger seas (and at greater speeds) than other SIB's.
We enjoy the rough stuff (that's me and my 12 yr old son - wife won't go to sea, rivers only
) but if you want to take non-boaty people out or only go out on rivers/flat days/enjoy more gentle prevailing winds then any typical 3.4m SIB will be great.