Huw,
Some basic points to keep an eye on whilst owning the E39 M5 are the MAFS ( Mass Airflow Intakes), a very common problem and about $250 to replace but is a very simple DIY job. You can test by accessing the secret menu (have a search to find out how to access this) and choosing option 4 (I think), good MAFS should be given a reading of about 140 or more simply engage sport in 3rd gear at about 3000rpm, the car should feel like it lurches forward. The thermostat is also poor, if the water temp gauge doesn't get above 1/3 it's gone, budget about $170. The VANOS can rattle on idle, this is not an issue apart from the noise itself and disappears under load however it is a nasty job if you need the vanos repaired or replaced.
They were facelifted in 2001, a lot of cosmetic stuff like angel eye headlights, new rear lights, 12:9 wide screen monitor in the dash, grey tachometer dials, it looks generally fresher all round. The VANOS was also modified which makes it more reliable and efficient. Might be worth doing those upgrades above to yours if you want to freshen the car up a bit and make it look much newer. Some examples also had black tachometer dials which looked really sharp in the interior.
Enjoy the 400hp..........
I will give you my dads BMW mechanics number who is mobile which means he can come to your home or office. He worked for Canterbury BMW as the head mechanic for for than 18 years now. I have referred him to Dimi already so if you want to be next let me know. He knows these cars very well and can tell you all the faults and problems to watch out for and what preventative maintenance should be completed to avoid issues. I think you should get him to do routine check on your car especially since you have just bought it to see if anything needs replacing.
Also, remove the rear silencers from the exhaust, the noise is amazing and it doesn't drone. Only problem is it can frustrate some owners such as Dimi who had his stock mufflers put back on due to the irritating and booming noise.
I also recommend you have a look at this website in your spare time as it provides such insightful information through not only videos but through detailed DIY's.
Expensive jobs are front discs and pads at around $2000 depending on whether you go aftermarket or OEM but I recommend you purchase all your parts overseas if possible. The clutch and flywheel are over $1,000(clutch and flywheel can last anything from 10,000-120k kms depending on how you drive it). The tyres are expensive if you go for top brands, but I've always found having seen the brands my dad purchases you shouldn't go wrong as long as you don't drive it like you stole it. If you need to upgrade the tyres I have no hesitation in recommending Michelin Pilot Super Sports.
Generally speaking the early cars can have pretty horrendous oil consumption, and they drink Castrol TWS Motorsport 10-w 30 at $31 per litre. Cars built after March 2000 have redesigned piston rings which slow the oil consumption quite considerably. If you want I can recommend you a place who sells TWS Motorsport 10-ws 30 for $16.00, massive saving.
Last but not least I also recommend you buy a BMW Peake code reader to check for any fault codes if they occur which is a handy tool to have for the car.
If you want to have a photoshoot done, let me know as I have close mate who is professional photographer and just spoke to him and he said he would be more than happy to do a photoshoot. So keep that in mind. Crisp photos of the car, surely thats indeniable.
http://www.m5videoguide.com/blog/
http://www.bmwtips.com/tipsntricks/
http://www.drvanos.com/index.php?option=com_content&view=category&layout=blog&id=4&Itemid=5