Off the top of my head you would need to get:
- Turbokit (Turbo, downpipe, intake/filter, turbo manifold)
- Oil feed + return lines + drill your sump to insert AN adapter fitting for return
- Intercooler (FMIC or SMIC will work) + Pipework
- If you are manual, then you will need an upgraded clutch as stock won't cut it
- Aftermarket management of some sort, piggyback or full standalone (GE Ecu can't handle boost) + sensors
- Probably a good idea to get an AFR gauge
- Bigger injectors
- Wastegate + wastegate piping
- BOV (optional)
- Engine Oils/Coolant etc
You might also have to take into account that turbo kits for 2JZGE on a supra, might not be a 100% fit in the GS300 engine bay.
You won't be able to achieve any of this for £300 lol. If you do the work yourself, then you can maybe get away with 1.5k-2k on parts, if you find some second hand deals or want to risk using china crap. If you need someone else to do the job for you then budget another 1-1.5k for labour.
All of the above takes is based on the idea that your current engine is in good condition, and dosen't need any seals replacing or service parts, otherwise that will be even more money to spend.