I am kind of new to this forum but not new to older BMW ownership. I have been driving and fixing them for 12 years. Prior to that, I spent a lot of time and money on Asian AWD Turbo cars. I currently own a 97 328i Convertible, 95 525iT (Touring or Wagon model), and 03 530i 5 spd sedan w/Sport package. I am telling you what I own so you know what my opinion is based on.
The earlier E34 525i wagon with an automatic transmission has just enough power to be fun some of the time and it will cruise all day at 80 mph. It will not win any drag races with the lumbering SUVs that seem to be everywhere today. This car is about highway cruising, not getting away from stoplights ahead of other traffic. The E39 525i wagon will do a little better because the variable valve adjustment (VANOS) will give it more low end torque than the older E34 engine. It will still be seriously underpowered compared to most modern cars and SUVs on the road today. The addition of a manual transmission wakes these cars up a lot. When I converted my E34 touring to a manual 5 spd it transformed the driving experience in a very positive way. I am sure the manual E39 525iT is also much more fun than the automatic version.
I like the driving experience of the E34 touring so much that I have purchased a used S52 engine out of a 99 M3 and am in final stages of swapping that engine for the 2.5 liter that originally came in the car.
FWIW, I bought my E34 touring in Indiana and drove it east over the same mountains that QSilver7 mentions. It was good in the mountains at highway speeds but not outstanding.
Now to address your question about maintenance. The E39 touring models have the a much more complicated engine to maintain and service compared to the earlier E34 cars. This is true whether you buy the I6 or the V8. How well the car was maintained prior to your purchase will determine how much time you spend under the hood. Any E39 with 100K or more miles that does not have a well documented maintenance history is probably going to need $2-3K of remedial maintenance in your first year of ownership. That cost is with you doing most of the labor. You will need to address the crankcase ventilation system because it is prone to failure with both engines. Less so with the V8 but still a liability at high mileage if it the original. Shocks will be thoroughly worn out after about 80K miles but most E39 models I found on the used market were still on their original shocks. The cooling system requires overhaul at this mileage if it has not already failed and been rebuilt. Rear subframe bushings on the touring model have all failed by 100 K miles and most owners I spoke to played dumb and ignorant about this when I asked them. Power steering leaks from the hose under the reservoir are common. Oil leaks from a couple of locations are common on neglected engines.
Bottom line is expect a lot of maintenance actions if you don't find a car that has been well cared for and documented by the owner. Pay a little more for such a car if you do find one. You will not be likely to find one at any used car dealer lot.