Hi Newboy, Welcome to the site.
Very good question - which raises several points.
This is the way I would see the situation. You have an 18' boat , a Shetland (fairly heavy design) so a small aux. say a 4hp, wont really be of much help - even on a 16' dingey they are only good for emergancies, and for aux. propulsion you wont get abpve 2/3 knots max, in slack tide. So to give any really assistance - youll probably need about 10hp - ideal as this is what you have.
You mention a seagul. Great aux. engines, although any outboard doesnt like being laid down, and stored flat - so I would advise against this.
The 10hp you already have is in great condition and as you know it, should be realiable (this counts for alot with engines)
My own boat will raise the jockey wheel if I jump off the back whilst on the trailer, so dont be too alarmed - the real test will be to fit them and see how she sits in the water - swamping over the transom will be you biggest risk. If she sits fine - then in my opinion, whack the trusty 10hp on the back.
Cant help with the model - Im afriad, Try a good search of the web.
Hope this helps
Adam