Both of my Trailing Arms have a few small rust through holes in them.
However, I hit a 3 inch high island in the Road and cracked the Drivers side Trailing Arm.

I had right and left Trailing sent to me UPS from a Yard in my State. I installed the one to replace the cracked Arm just as it came to me and that was over 3 years ago. I have had zero issues with it.
I did not replace the Bearings in the used Arm I put on the Car. I never changed that other Arm and am still driving around with the tiny holes in it.

I had saved the old cracked Arm and eventually bought the Tool needed to remove the Nut on the Arm.
I Pulled off the Hub using an old Rotor to wack on (instructions in our DIY Section).
(It looks like any method of removing the Hub destroys the Bearings Good or bad.)

With the exception that I had more room to swing the Big Hammer removing the Hub would have been easier if it was attached to the Car.
It should not matter much assembling the new Bearings until you reach the point where you need tighten the Nut and set the Hub Clearance/End Play.
setting the End Play may be easier if the arm is attached to the Car.

You certainly cannot put the Welded Tubular parts of the Arm in a Vice because they will bend/crush if you put a lot of force on them.

Since I already did it; I would use the Arm just as it came from the Junk Yard and see if the Bearings are still good in it.
If you decide to replace the Bearings I would buy them youself so that you get good quality ones (it is too much work to replace them).

If you bought the Tool and an old Brake Rotor you could pull the Hub off yourself and you could see the Bearing numbers and go hunting on Ebay for some USA or Japan Bearings.

__________________
84 300D, 82 Volvo 244Gl Diesel