Thanks for the advice guys.
Surely buying the bushes only will incur more labour charges as the components require work on the bench, so you could end up with the same sized bill whether you do the bushes or the arms? I get that doing the bushes yourself would be far cheaper, but I just don't have the time anymore - kids, job, degree study, soft hands from lack of practice......
I strongly suspect the best thing to do is replace all 4 arms, then one can pare back depending on budget.