Get ya...
If it was my money I would still opt for the Ultra SRs, can't fault them for the money and are probably better suited to our roads than the STI shocks. Also 2nd hand shocks is hit and miss, with new KYBs you know where you stand.
Here are a few pointers based on what you wrote:
ALK is good and worth having -- as are different poly bushes for things like wishbones at the front and trailing arms at the back.
NOTE: If you are lowering your car you Really need to think about fitting a Roll Centre / Bumpsteer kit (same thing different name).
When you lower the car it will result in significant dynamic positive camber due to the way the Impreza suspension works, the above kit makes the needed changes to eliminate this... would not recommend lowering without fitting this as a lowered Impreza without a bumpsteer kit will grip will less - in fact grip will be bit scary as it will initially bite into a corner and then as the suspension compresses it will gain positive camber and your car will suddenly wash out with understeer as a result of the sudden loss of negative camber.
Rather than fitting a different ARB I have found that fitting uprated droplinks on the back (the solid alloy C shaped ones) works better (and probably more cost effective too). The plastic oem ones can break, but also they are weak and take a while to load up the ARB with tension when you are cornering (so takes a while before the ARB can even start to do its job regardless of its size). The more solid droplinks load the ARB immediately when you corner, so no delay -- especially when changing direction a few times in quick succession (think twisty back roads) this simple upgrade makes a huge difference. The uprated ones make the car much more controllable.
Just fitting a heavier ARB doesn't get round this issue and also if you go too heavy with the ARB you again will lose rather than gain grip. At the front you are probably better off leaving both the ARB and droplinks alone. The OEM front bar is actually sized to reduce grip and make the car "safer" by inducing some understeer. So one can either improve the back (which is recommended) or put one size smaller on the front to make the car more neutral compared to the factory handling.
Keep in mind that when you get better shocks and (later?) stiffer springs, the car will already corner much flatter anyway as the ARB is only one thing that controls roll during cornering. You need to evaluate springs, shocks and ARB with droplinks all together to understand the resulting handling behaviour.
Hope the above helps you a bit with the planning.
