If you're looking for good information about spotting at BRU airport, check out http://users.telenet.be/PowerCorruptionLies/BRU.htm
Concerning your examples: view from the WingTips restaurant used to be pretty good, but since they built the new Pier A, the view you had of RWY 07L/25R has mostly been obstructed. But you can still see aircraft taxiing on the ramp between Pier B and Pier A. I'm not sure if you get a good view of DHL freighters from the WingTips restaurant but I wouldn't be surprised if this is the case (DHL is located to the left of WingTips restaurant and since most freighters are located somewhat in front of the DHL buildings, it's possible to get a good view of them, especially when they taxi (not sure of this, so don't shoot me).
I can't tell you all the different focal lengths you would need; this depends on the distance between you and the aircraft. What I mean is if, e.g., a 737 parks just next to the WingTips, you will probably need a wide angle, but if a 737 is taxiing at the other end of Pier B, you will need a good tele. This is also the case if an aircraft takes off from RWY 25R for example: a 50mm won't do the job, not even on a 747.
Concerning the other spotting locations, it can be quite difficult to access them, especially if you can only rely on public transport. However, I was told that one of the most famous spotting locations is accessible by train: you have to take the train from the airport and try to make it to 'Kortenberg' station. Also, it is possible that you can access some good locations by bus (there are lots of buses that depart from the airport terminal), but I don't know their exact routings. You can always try to find some addition information on http://www.delijn.be
or send an e-mail to email@example.com
Hope this helps!