that's a lot of gear.
If you want to store all that you'll need throwover panniers as well. Plus aside from a top box, your kit will be insecure.
A top box is quite steep most of the time, assuming you don't have a mounting plate. You'll need a plate, an adaptor and a box. Some boxes come with adaptors. Also consider how big it'll be. Some are almost wider than the bike (might even be wider given a 125s size).
My personal suggestion is to always carry your helmet with you - if someone wants your bike and they take it, they'll often smash open the top box and there you go -everything they need to look legit whilst riding past cop cars on their hotwired bike.
The jacket is a pain to stash as it's easy to squish the armour. Plus it's easier to get off it with all your stuff in your pockets.
A tank bag is fine if it's small as bigger ones get in the way of arms and instruments. The magnetic ones should be used with care as they'll pick up any small bits of metal (they're everywhere!) and scratch your tank to hell so it's best to put some cloth or tape between magnets and tank. I got one for £30 off amazon and it was just fine.
Throw over panniers are great if your exhaust is out of the way. Oxford ones are apparently good but after experiencing an expensive oxford tank bag where just about everything that could break broke I'm unsure about the brand. Remember that anything you put in here is insecure. Boots are smelly and therefore safe.
Downside of throwovers is that they make the bike wider so you can't get through the gaps you once could.
Ideal combo in my view would be a top box (smallish) and throwover panniers. You could put your hat in the top box, boots in one pannier and the trousers in the other. I'd still be wary about leaving expensive motorcycling kit insecure and one day some kid will think it's a great prank to steal your boot.