That e39 520 does look nice (and comfy too). Funny how I say that now while at the time I hated that model (well especially the back as the front is nice enough). I suppose it has aged well (maybe due to disappointing looks of modern cars??).
As for running costs... yes little age related things will need doing, but that is nothing compared to depreciation costs on a newer vehicle -- and... doing those things, if well documented, only add to the value if ever you wanted to sell it again. At worst any such work over the duration of ownership will be cost neutral - unlike a newer car that will also need bits and pieces and will still depreciate on top of that (unless brand new, all cars will need stuff like droplinks, bushings, brake hoses, bearings etc etc during ownership if holding on to it for more than a few months...). As Colm pointed out, that model is relatively simple and reasonably reliable (especially when compared to the horror show that is today's BMW reliability record).
On that model 5.20i though... having driven several back in the day as rentals and friends' cars... it is ok to get around, but personally would class it as pretty gutless power wise for a Beemer - to the point where it will irk you before long if driving it as a daily. Maybe that is not a concern for you as you have the Scooby for power fix etc, but just be aware it is a bit sluggish unless caned and even then it is not exactly fast.
Another thing... I am a great fan of LPG and have owned several LPG powered cars... however... BMW engines do NOT like LPG. Some are fine on it, but many develop issues with headgaskets and burned valve seats. Also, LPG conversion is going to set you back 1500-2000 on a six cylinder and unless you do savage mileage, or plan to hold on the car a good many years it is not worth it. Another thing to consider is that the LPG conversion would hurt future "classic" values if selling on.