I used to sell and work on 70s Motobecane mixtes. They all had lugged frames. Find out if this is the Nomad model or the Mirage model. The Mirage was a higher level bike - better frame, better components. I'd pass on it if it is a Nomad. The shifters are on the stem, not the head tube. You won't be able to get indexed shifting with this bike. It has a 5-speed freewheel that won't work with any indexing system. You could install Suntour bar end shifters without indexing - they are very nice. Check whether it has a cottered steel crankset or a cotterless alloy crankset - could be either depending on year of manufacture. I'd probably pass on it if the crankset is steel, unless you really want to invest time and money upgrading this bike. But parts will be hard to find because French threading is no longer in use, and cotter pins could be frozen and impossible to remove. Check the rims for trueness, dents, and flat spots. Check that the hubs are smooth and the rear axle isn't bent. The Suntour derailleurs should be good for many more years. Diacompe brakes were never my favorites because they always feel rather mushy, so squeeze them hard and make sure they stop adequately. If you can get some pictures of this bike, maybe I could tell you more.
Oil is good, grease is better.
2007 Peter Mooney w/S&S couplers/Terry Butterfly
1993 Bridgestone MB-3/Avocet O2 Air 40W
1980 Columbus Frame with 1970 Campy parts
1954 Raleigh 3-speed/Brooks B72