i have an 84' 280e euro,,,i've used the haynes usa manual almost exclusivly, ( which bites because the the 123 280 wasn't produced in an american version in 84', so my year isn't even covered by the haynes), i purchased the factory manuals on fiche as i was told it was the only version available for my model. i use a professional quality reader printer and the fiche are still very hard to read, and the layout is very confusing. it's about a 30 fiche set and it cost a little over two hundred. unless your going to do major work, ie... an engine overhaul, or something, i would just use the haynes american one. the information is partial at best but the pictures are good and fortunetly the engineers at mb have helped by making the car very mechanic friendly!!!
the only problem i have had is with local parts stores, ie... auto zone, they carry many parts for my car believe it not, but if i tell them i have an 84' 280, they look at me like i built it myself, i've learned that to buy parts at places like that i just tell them its an 81' 280, ( the last year they made a us version)
this may not help you with your 126 car, i thought an 80' would have been a 123 like mine, i think my haynes covers 80's but with the euros who knows!!!