One major issue (outside of the ones already mentioned) with old reefers/freezers concerns the seals on these units , as almost all of them are/get rather crisp .
There are modern 'solutions' to this , but all involve using modern seal 'kits' , which are not a 'match' for the era of the old ones ... if you're planning on making a keg-cooler , perhaps this is not so much of an issue for you ...
All refrigeration units function via removing heat , so leaking seals (in/on the doors) will make the old compressor work overtime & lead to larger problems .
As others have mentioned , transporting/storing a fridge on its' side will cause the oil in the compressor to migrate into the coils of the system & can/will turn a working compressor into toast , in short order !