Fast enough depends on what you want to do. If you are just puttering around small lakes and ponds, then you will be fine. Lots of lakes in PA have 10 hp max restrictions & you will see 14' & 16' semi-v's running 10 hps all the time. You wont go anywhere fast but you will get there faster than a trolling motor. Depending on the load, it may even get you up on plane & scoot right along.
Now if you are fishing big lakes & swift rivers, then the 10 may not be enough. If that is the case, then a 25 is more like it.
So, decide where you will be boating & then decide which level of hp you will need. Then see if you can beg, borrow or steal (well not steal) a similar enginer to do a test drive.
One thing to remember, is that if you get in a tight spot, you will (almost) never say "wish I had less HP" but you can dang sure get into situatoins where you wished you had mx hp hanging on the stern.