Hi,
You have a number of options, each with it's own advantages and disadvantages.
Retrofit all the oem modules but with the widescreen monitor, Mk4 DVD based GPS computer, oem bluetooth, add an iPod interface and accessibility to the car's onboard computerized functions via the excellent Intravee 2 unit. Advantages are all the oem looks and full access to the OBC, disadvantages are very expensive, old 11-15 year old modules with the higher risk of failures, outdated technology, potentially limited bluetooth connectivity and oem levels of sound quality.
Fit a premium brand, high quality aftermarket head unit, either single or Double DIN. Advantages are a new current technology unit, all the latest features and functionality, better sound quality, excellent value for money. Disadvantages are not 100% oem looks in the dash, no full access to the car's OBC.
Fit a low(er) quality cheap chinese replacement head unit. Advantages are that it'll physically fit into the dash aperture and provides a psuedo oem'ish sort of look and that's about it really. Everything else about them is low grade and quality, often with reliability issues, potential features not working out of the box or cease working fully or are intermittent over time. The 'best' choice out of these type of unit's in the UK is the Andrive unit which has full warranty and support from the company in case of issues, a better quality unit compared to the other branded and unbranded unit's out there and some exclusive functionality as it's essentially more of a car pc wrapped up in an E39 sized chassis.
For me, the best option is the premium brand head unit option - you get a quality unit for your car from a recognised and reputable brand. I've mentioned it previously here on m5board, in my E39 Touring, I have the Parrot Asteroid Smart unit and the writeup for it is here;
http://www.m5board.com/vbulletin/e3...allow-depth-double-din-android-head-unit.html
I'm sure other's will perhaps add their feedback as to your choices too!
Cheers, Dennis!