You really need to decide on what boost levels you will run and maximum intended rpm.
Then you can size the turbo based on maximum air mass flow and boost pressure.
HKS GT3037, 3037R and 3040 all (I believe) use Garrett T04S compressor wheels of different trim values.
HKS GT3037R 450hp rated Compressor 52 trim, turbine housing 1.01 (14cm^2) A/R
HKS GT3037R 480hp rated Compressor 56 trim, turbine housing 1.12 (16cm^2) A/R
HKS GT3040 500hp rated Compressor 50 trim, turbine housing 1.12 (16cm^2) A/R (not T04S???)
All have exhaust wheel diameter of inducer/exducer - 60mm/55mm
Also non ball bearing
HKS T04S 550hp rated Compressor 60 trim, turbine housing 0.84 or 1.0 A/R
(Has bigger dia turbine wheel and no BB so more lag - 74.2mm/64.7mm)
Garrett also do ballistic GT series
GT30
PART NO Compressor trim Wheel diameter Turbine A/R trim Wheel dia.
705867-0001 56 T04S 2.25 /3.00 500hp 0.63 84 2.16 /2.36
705867-0002 56 T04S 2.25 /3.00 500hp 0.82 84 2.16 /2.36
705867-0003 56 T04S 2.25 /3.00 500hp 1.06 84 2.16 /2.36
When choosing turbine housing A/R it's all about trade off between spool up (lag) and flow (top end power).
For least lag - small A/R. (Also small dia. turbine wheel)
For best flow and power at max. rpm - big A/R
If A/R is too small then turbine temperatures can also be too high as flow is restricted of hot exhaust gas.
Also consider the Trust/Greddy turbine kits as used by Jun themselves - td06-25g. (A Mitsubishi turbo)