If you can afford it, stick to the big names - Clearcom and Techpro for example - or at least a system which is compatable.
The TD900 Series is decent - for the price - but it is all proprietary - including the connectors between the headphones and the device. It does not play too well with other systems.
Also - there are very few times when someone will need to be wireless, so I would look at a wired system - you cannot beat wired for a reliable system. Then I would get one or two wireless packs added to that if needed (you will probably find it won't be required).
The major problem is frequency. 2.4gHz is the frequency used for most of this stuff, and there are limits on how many items you can fit within the 2.4gHz spectrum without interferance. That means that the more wireless stuff you use, the more chance of interferance. Wireless microphones, wireless cans, wireless networks, bluetooth - all likely to be within most venues. Things get crouded pretty quickly.