Ring 16 de winterise questions

[CSLmarson]

hi there

im new to the boating scene so if this is daft dont be to harsh,

iv recently brought a ring 16 with 115hp 1998 merc

happy with the boat it needs a bit of tiding but nothing to major.

i have a few questions regarding the engine, im going to change the impeller and gearbox oil first thing and im told the engine had been winterised so my question is do i need to de winterise it or do i prime it and it should be ready to start ?

are there any major items i need to take care of before heading to sea ?

im also going to do a compression test on it what kind of readings should i be getting across the cylinders ?

Also what size battery do I need ?
cheers