Hello Suzy Q,
I attach a typical picture of long tails on a beach near Krabi , you will note the different sizes of boats and also different sizes of engines that are balanced on the transom.
To operate the average Long Tail requires quite a lot of upper body strength - first, if the engine is a former rice tractor engine then it will be a hand start type - which requires winding the external flywheel with a crank handle with one hand while the other hand decompresses the exhaust valve. Once started, the whole engine is swung on its transom pivot, then the prop is dropped into the water to provide propulsion in a required direction.
Bigger engines (up to 8 cylinder) - also balanced on the transome are started electrically - these require a lot of strength and EXPERIENCE , certainly not for a first timer.
A small boat with small engine with long tail propulsion could be found 2nd hand for around
60,000 baht - at current rate +/- US $ 1500.
I think the best way to go would be to just find the boat - 25ft thereabouts with suitable transom for an outboard - then find electric start between 15 /25 hp . Design a sun awning - hire a recommended driver - take off - explore Phanga Bay.