I prefer using outboards on all my own boats. Simpler, no space lost inside, no smell inside, it can be taken off and taken to a mechanic for repair, not vice versa. The only real drawback is you get no "free" hot water via a calorifier attached to the diesel engine.

Typically for boats under 8m (26ft) a 4-6hp outboard is enough, always get a long or even extra long shaft. A 2.5hp outboard is plenty on the Strike trimarans and Chat18. For larger boats use a high thrust outboard, those made by Yamaha are perfect. A 8hp on 8-9m (25-30ft) boats (Gypsy/Saturn etc) and a 9.9 for boats 9-10.5m (30-35ft) If you normally keep your boat in a marina or have a heavier boat then twin 9.9hp make sense, although twin outboards use more cockpit lockers (extra fuel tanks and batteries) never mind extra weight.

There are several options for placement. On smaller boats (Strider/Wizard etc) you can fit it on a bracket on the aft beam. I fit the outboard on the starboard side as I am right handed and that makes it easier to pull start. The major advantage is that the outboard is then out of the cockpit area so it's great when sailing, nothing to catch ropes etc. Disadvantage is the prop may aerate when motoring in a chop, while refueling and, especially, maintenance can be challenging while hanging over the water. Your choice, for sailing its better on the aft beam, for motoring its better in the cockpit.

The best solution for larger boats is to fit the outboard in a nacelle.

I usually draw the nacelle at the back of the cockpit. It's out of the way, the engine can easily be steered, it can, if necessary, be hand started. However deciding on the nacelle height is always a tricky one especially as you often don't have an engine to try it on, and even if you do its heavy and awkward, made worse by not having a nacelle to fit it on. I often wish engine manufacturers sold/rented a styrofoam mockup!

The crucial distance you need to know is the longest distance from engine pivot point on the bracket to the back of the cowl as you don't want it to hit the aft beam. That will determine the top aft corner of the nacelle. The you also need to know the height of the transom. I usually have the bottom edge about 200mm min above the WL and a transom about 200mm deep, thus the top of the transom is about 400mm above the WL even though the leg is 20in that's OK, the deeper the better for a catamaran, the higher the better for a planing speedboat, so don't get misled by some dealers. The transom itself should be angled about 12deg, as on speedboats, so that the engine leg is vertical or even angled forwards. It is easy to rake the leg further aft, but you cannot ever rake it forward.

Those two measurements will give you the distance forward of the aft beam.

But as those distances will vary from engine to engine, it's impossible to say what yours should be I'm afraid.

The photos show some options, both for a single and twin engine installation. You will see ropes in the in-water photos. These are of my Eclipse. I use a rope to pull the outboard up and another to hold it down.

That saves me opening the outboard locker, pushing hard on the catch, barking my knuckles etc to release it. Then struggling to pull the engine up and then doing the reverse to lower it again.) Note the pull down rope has to take the engine thrust when in reverse so a 2:1 purchase makes sense and make sure the cleat can take the load (I use a conventional horn cleat)

The other two ropes got from engine to tillers and, when tight, mean the engine turns with the rudders (obviously slackened completely when sailing). You have to fiddle with the positioning as the rudders need to turn further than the engine so you don't want the engine limiting movement. That helps manouvering, epsecially in reverse. Its optional, not essential.

The final photo shows the low-down control box (the cleat is the horn cleat for the outboard hold down rope). Out of the way, no chance of ropes catching on it and less bends and shorter cables needed. Someone saw it and said "why do you control the engine with your feet?" I said  "what do you do in a car?"