The w123 filter is not really that big. It has a sort of "bypass" that is just cotton gin waste to provide surface for soot entrapment. The actual filter is quite small.
I recommend mann, hengst or mahle. US-made fram has been found to be made quite well, and you really cannot go wrong with any of the majors, but some may not come with all the o-rings and washers. Mann costs $12.30, Hengst $9.45, Bosch $9.58 at my favorite MB parts supplier at peach parts.
FYI, the full-flow portion is only the yellow media at the bottom/front (in this picture) of the filter... Not much:
I personally would not be opposed to using any quality filter for 10k. The only issue is soot loading. You can argue that the "bypass" isnt all that great because you will get to 2% soot loading in about 7500-9k miles (if you didnt change the oil, you will of course), of course these IDI engines can be fairly sooty, especially if the injectors are not balanced and valves adjusted... so it may be quite good too, in that the filter absolutely protects from excessive soot for >5k miles. That said, there is no easier oil filter changeout than on this car, and I would do it each time myself.
I play the resident w123 expert on here, and lots of folks on here have experience with these cars. We need to see pics. The 85 is a different vehicle in many ways than the other w123s, and I can help to walk you through the differences if you like. Ask any questions and I'd be glad to help, I currently own two of these cars myself...