Hey, welcome to the forum.
1). Engine swap is always a fun thought but can quickly become costly. Make sure you know what you're getting into. If you choose to proceed, Monkey Wrench Racing has some good swap kits. If not, consider a standalone ECU like the Apexi Power FC.
2). I wouldn't run any taller than 18" on a Celica. If you do, you may risk destroying your fenders. You can go wider though. I'm running 17x8.5 and it fills the gap well. The side profile looks good too, very flush.
