Min / Max
I had both a 9.9 long shaft (Johnson), which was stolen, and an 8 hp long shaft outboard on my old L25. The 8hp Evinrude was fine for routine use, but the 9.9hp was noticeably more powerful and would be an advantage if motoring against a current. The extra long sailboat shaft length is a definite requirement if you are on the ocean, as even two foot chop will have the outboard out of the water at times. I wouldn't recommend less than 8 hp. From what I have seen the addition of the ability to charge off the outboard pretty much starts at 8hp too.
The biggest plus to the 8hp was that it was MUCH easier to haul around than the 9.9hp, which is essentially a detuned 15hp (different carb). It doesn't sound like much but when your boat is on the hard and you are lifting 80 lbs vs 100lbs over your head to place it on the transom mount it makes a difference. A 4 cycle may be even heavier.
Also, a transom mount is the furthest aft point on the boat. The L25 is a light enough boat that she will be sensitive to weight aft. Not that sensitive, but more weight aft may affect trim. Watch for where your cockpit drains are and stow some weight forward if they are close to under water at rest or with your normal load of crew. The drains can get very close to the water otherwise. Be sure that the drain hoses are crossed too (port scupper to starboard thru hull and vice versa).