Gday Mikey
You can do several things, you can change just the ceramic wheels to metal ones and keep them stock size but of course no power difference except the ability to boost the new turbo more. Or you can Hiflow just the front side or hiflow front and backside with your housings machined out like you were saying.
Both of your wheels will have to be changed-ceramic turbine wheel and plastic impeller wheel to metal. You cannot repack your turbo bearings the turbo mechanics will replace it with a brand new CHRA unit
This may cost you the same as buying one off the shelf for ex from gcg.
If you go a garrett 45hp turbo or similar u will have a rude shock if u think its only going to cost you an extra 1k when you will be forced to change your dump pipe, Compressor Intake pipe and intercooler pipe to fit, then there is water lines and oil lines which now dont line up.
Anyhow good luck with your choice
mac