I fitted one upper arm at the side of the road last week for a local member.
The job was made trickier as he had attempted to undo the pivot bolt with a bi-hex socket and knurdled the head.
Removal of the old arm, and fitting of the new one took me around an hour and a half, on my back with the car on axle stands.
If we break down an estimated £700 bill, a pair of arms are £240 (retail), and on a ramp the job will take no more than two hours assuming the rose joints are OK, and another hour if both rose joints need to be replaced.
So a maximum of 3 hours labour, and a maximum of £300 in parts........err I would look elsewhere if I were you
And no you don't need trailing arms either
Brian