Oh no..did I do that?

I'm just saying think about it. Doing a CA18DE conversion is a great way to upgrade to a DET, when you turn 25, or when you can afford it, whatever comes first. If it's your only car, then you'll need to think strongly about the impact of having your car off the road for a couple of months.
Is this being done on a Uni Student budget?

If you are needing a higher top speed, then best thing you could do is fit a 3.5->3.7 diff.

Would cost you about the same as the gearbox, and if you fitted a bigger diff, you'd be ready for a CA18 future. Buying the dellow and celica box will tie up a fair bit of investment in the A-series combination. Go for brakes and diff first. That way you'll be safe, get the improved cruising ability, and be on track for a future CA conversion.

And if you get sick of the 120Y, and decide a 1200 would be better, you can transplant across the parts...handy!

Chris.