I would not recommend used lift arms. They are fairly easily broken, and who knows how close the used pair are to breaking? This is not an impossible job, but it's not an easy one ... you don't want to do it again when the used set breaks. However, it's probably luck of the draw ... if you can find a set that looks good and from someone you trust has cared for the sunroof (that makes all the difference), go for it.

I do not understand what "the whole assembly" means. The whole sunroof? The lift arms and slide rails together? If it's the lift arms and slide rails, they may be right. I'm not sure if this applies to the 126, but in the 124, if you're car was built after a certain point, it has updated slide rails, so you're fine. If it's built before that date, you need to buy the new slide rails to correspond with the new arms you'll be buying. However, I don't recall the rails being very expensive ($20 each?). What did the dealer quote you?

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1987 300D, arctic white/palomino--314,000 miles
1978 240D 4-speed, Euro Delivery, light ivory/bamboo--370,000 miles
2005 Jeep Liberty CRD Limited, light khaki/slate--140,000 miles
2018 Chevy Cruze diesel, 6-speed manual, satin steel metallic/kalahari--19,000 miles
1982 Peugeot 505 diesel, 4-speed manual, blue/blue, 130,000 miles
1995 S320, black/parchment--34,000 miles (Dad's car)