Trailbraker

You can do either, but I strongly recommend buying them pre-polybushed.
Besides it being the most convenient , its apparently a fair bit of labour to get somebody else to do it afterwards.
If you want to do it yourself fair-enough.

However bear in mind that it took some retailers of poly-bushed wishbones a significant time to identify long term issues with wear due to some polybushes being a very slight bit too big.

I got mine from Autodelta, they were superb, no long term issues and superb performance.
(Got the rears done first btw, expensive but excellent)

I'm not a doctor - but I really do recommend braking later