hello,
you have really asked how long is a piece of string !! there are many answers, some work better than others
my first suggestion would be a TVR-1+centre flue boiler w gas fired burner, there are some available comercially
there are also engines available from other suppliers,slide valves are more powerful than ocilators, but need 2 servos , throttle and reverse valve control,as i said earlier, many answers, sometimes you just need to suck it and see
a TVR-1 will drive a tug ,w a 75mm prop, up to 1000/1400 mm , a small engine is very powerfull.
if this is your first model,I would recommend going w electric , it is much easier, and reliable
bills