your only going to need a engine and radio to get the arf ready to fly, as long as you get a OS 46 2stroke or eq.

the engine mount that comes with the plane is designed for the OS 46fx/AX either engine will fly the plane well,you might get a os 55AX to fit as it has the same foot print if you are looking for the extra power for the float plane option.
but you will have to shift some radio parts around ex. the battery,and reciever etc...to get it to balance the plane was designed with the 46 in mind.

I am in the middle of a NEXTAR float plane conversion, and upgraded to the OS 61fx but I had to change the engine mount to fit the larger engine,I just happen to have it handy so it's the one I decided to use.
as well as the gear relocation and some minor repair work that needed to be done after some beginner crashing, I am now recovering it after fitting the floats.
a couple of photos before the recovering, the fuse and floats are now white with red trim.
I liked the NEXTAR because it didn't look like a box and resembled a real plane.