JSPOC is currently tracking around 22,000 objects, at least, unclassified objects. That number is fairly low actually, compared to the vastness of space. As of writing this post, Flight Radar says there are over 5000 planes in the air, and it's quite early on an East Coast Saturday morning. Also adding in the fact that most planes are larger than satellites, and that satellites are uniformly spread across the globe, while planes aren't, and you've got somewhat of a picture as to how rare space junk is in space.
That being said, how likely is it that you will have a close encounter with space junk? Not very likely. From my experience, a single satellite will pass within a few hundred meters of a trackable piece of junk only once maybe a few times a year. Then keeping in mind that you actually have to be much closer in velocity to take advantage, and, well, you've got a lot of problems. You might be able to latch on to a larger piece of space junk and use it for some purpose, but the material wouldn't be ideal, and the orbital rendezvous problem is difficult still, although it is manageable.
You probably could de-orbit a spacecraft by sticking out some large wings meant to withstand the impact of small space junk, which would slow you down gradually. But the same effect will be had by air resistance.
Bottom line is, you're more likely to find use out of fixing old satellites, rather than using them for fuel. Use them for spare parts, putting in new batteries, fuel, etc. But you're probably not going to get anything significant out of them in terms of propulsion.