I went through this with my son on his (formerly my) 1986 190E 2.3-16. He replaced all four fenders and the underbody (floors and rockers). He did some of the preparations and reassembly, but the underbody parts were not the heavy cost. No real finishing required - just paint with POR 15 and then undercoating and painting the inside - so it is not the time consuming and costly part of the job. Even the body parts were relatively less costly than the more visible fenders. The most expensive stuff, by the way, were the plastic spoiler parts.

Once you get going on one of these jobs though you will find yourself changing brake lines, and doing some rewiring. All good stuff and stuff you can do yourself.

This all assumes the stuff that looks good is good. I would caution you to check that out carefully especially around the wheel wells.

But a car with no rust from MB's W123 line is something special. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)