As another teenage (well, 21 years old...but whatever) car owner, my best advice to you is this: keep it simple.
220,000 Kms. isn't TOO much for a car, but things are prone to break out of sheer age. Simple stuff, but stuff that you'll have to take care of and that means money out of your mods budget. Gaskets, wiring, trim, etc. When I bought my car it was 5k kms. shy of 200k kms. and it ran beautifully. Two months afterwards spark plug cables failed. Three months afterwards a fuel return valve. Four months afterwards the head gasket had to be replaced because it still had the one it came out of the factory with. You can't predict those things, but the moment they happen your car is gonna be parked and you'll want to fix it ASAP, can't do that when you spent all your money the week before on some coilovers.
Turbo? Do you really need it? Sure, MOAR POWAAAH is always cool, I feel that urge too, but keep in mind the old, wise saying: "300 horses poop more than 200", and the more horsepower you pack the poop grows exponentially. Plus, why not learning how to drive fast and then pumping more power into the engine? "If you're not fast with 90 horsepower, 900 aren't gonna be much help" Wanna know who said that? One of the guys responsible for the original design of the Miata. Power, at the end of the day, is just figures and a game of how much money you can throw at it. Power =/= Speed =/= Fun. If you only think about power, you're going to be miserable and you're gonna hate your car. Been there, done that.
Is it a car worth having and messing about with? Absolutely, every car is when you're young. Miatas are awesome and that's a plus. But the best thing you can do to your car is keep it running well, and then building on from that. Learn to live with your car stock, learn to love it stock.
My $0.02.