Yes, you are wrong - there is plenty of risk associated with installing parts where the prime consideration for purchase is "dirt cheap" costs.

Good luck with your plan but I'm guessing you are not going to be able to find an independent shop where you will be able to dictate the terms and conditions under which the shop owner will complete the repair.

I don't know about W123's but I've done a set of lower control arm bushings on a W124, several special tools are required. Parts alone were over $100 as I recall - I don't think $500-$700 is at all unreasonable for the job.