When I upload, it complains that it cannot find a timebase - I am sure that I named one
It is almost essential that you have a timebase named T_GLOBAL in the downloading file. T_GLOBAL is used against any range that is downloaded. Tmebases named generically as T_something only apply to ranges on the sheet that the timebase is - it is truly a local timebase. Sometimes people name a local timebase and do not understand why CFA will not apply it globally.

Are there any important set-up steps (as regards CFA) I need to take after installing Business Functions?
The most important thing is to specify the CFADATA directory, where download data is stored. By default, this is under the Business Functions directory, but that may well not be the best place. In the root of your drive eg c:\CFADATA is better, but if you are on a network, somewhere like Y:\CFADATA (or whatever your network drive is called) is the best choice because then you can share your downloaded data with other users on the network.

Why did you not use something like XML for the download format instead of the existing plain text format?
XML is only really useful if you are going to give it to generic XML readers, and XML leads to very big files when you are dealing with ranges, or long lists, of information. In short, it is overkill and you don't need it. The CFA data format is designed to be clear and concise, that way it is quickest to download and upload, and occupies the minimum of space on disk.