Between the 523i or 525i, go for the 525i. But if you intend to mod the engine sometime in the future, then go for the 523i as the single vanos engine is more receptive to tuning. Most efforts on the double vanos units are futile.
Parts are pricier as you are buying parts for a luxury barge that costs >RM300k when new so you cannot expect parts at bargain prices. Most people forget that as they pay the depreciated price for E39s. Consider what DD said about getting those about 5 years or a tad older as most of what would go wrong would have and would have been or will be replaced.
These cars wear very well and you can hardly tell the age of the car especially if you are buying from a dealer as they will virtually look brand new with a new set of leather seats. The transmission and engine is rock solid. Most of those UK imports with Nikisil problems would have been replaced back then so you have nothing to worry about.
If you are looking at the older ones, consider the 528i as it comes equipped with lots of options. Doubt that you would be happier buying one relative to a 523i as the 528i comes with electric memory seats, electric steering column, multi-function M-steering, auto-dipping side mirrors (parktronics I think), electric rear blinds, full on-board computer etc. It is quite a heavy car and the extra torque helps to move the car. Once you are in motion, the car is actually quite brisk. Won't have any trouble breaking 230 KM/H.
Handling is 5 star plus plus plus the comforts of a luxury barge. Somehow the aluminium suspension arms does wonders to the ride comfort even with 19" fitted. I have personally tried both 18" and 19" but stayed with the 18" M5 oris as they look great.
If you are a bit of a handling freak, you would be pleased to know that the car comes standard with a 25% slip LSD for all the fun stuff to break traction. The car is extremely chuckable at all speeds and is well balanced. You won't feel the size nor length (incidentally the Estima is 1" shorter). You can tackle twisties like a straight line. Just floor the pedal and let the electronics take care of it but do it sensibly as if you over do it, nothing can save the car. The ASC that comes with the car is pretty good so you don't have to worry about playing second fiddle to the additional DSC programme. Between you and me, I often enjoy flooring the pedal and watch the ASC light fickle and fickle to gain traction. One of the best quick fix you can get is to switch off the ASC and floor it coming out of a junction.....Smiles in all departments.
The car hides speed very well. Cruising at 80KM/H is virtually silent and the car feels as though it is hardly moving. Spike it up to 160 KM/H and it feels like about 80KM/H in most cars. Hence you can virtually cruise at 200 KM/H for hours without tiring out. Neither would your passengers know the speed as it just hides it so well. Kinna of good if you like to drive a bit faster and keep your wife/GF happy by the side.
Once the maintenance parts are replaced, you will enjoy miles and miles of trouble free motoring. I did my replacements over the past two years and the car drives like a dream now. Most people are baffled to learn the age of the car as it just age so well.
Again, the E39 is like your dream girl that hardly wrinkle with age so be really careful when you buy from the dealers as a 96 can easily pass off as a 2000. Some of the forumers will share their grief on that. A lot of cars are coming from Singapore as scraped cars. Most are declared with forged documents and are declared as 2000 cars (can't bring in cars older than 5 years under the used APs).
One quirk on the E39 that puts many people off is the missing pixels on the Multifunction display. That can be easily rectified locally for about RM1500+ or so (last I checked) on an exchange basis. Other minor gremlins are the rear cup holders that gets jammed all the time as people's feet keep hitting them. But these are things that you can live with as this is after all a second hand car.
It's a pretty heavy car so don't expect kancil like consumption. I reckon that the 528i is a tad more efficient than the 523/525 as the additional torque (280NM vs 250NM) goes a long way to ease off your pedal pressure to get the car moving. I reckon that an average consumption for town driving would be in the region of 14.5l/100 KMs and 12.0l/100 KMs for highway cruises. Plus or minus 1.5l/100 KMs for varied driving styles. Driven hard, you would probably expect 16.0l/100 KMs. On the highways, driving at speeds in exess of 200 KM/H to Ipoh, I regularly get hit a consumption figure of 12.5l/100 KMs. Torque helps heaps when you decide to cruise above 200 KM/H or at any speeds for that matter. Btw, stay away from Vpower as it screws up your sensors.
I could write pages and pages on the area of tuning the E39 so I would just keep that off this posting. let's chat about that once you get your car and off we go and spend some $$$ to quicken the pace. Mind you that the 528i is already a pretty quick car so hang with it for a while before spending the moos... Good luck with your hunt.