I think you (and other people that gave their advice on the transformer) should take a look again at the link to the PSU kit. I noticed that the website mentions:
Supply voltage: 18V AC / 200mA
It also mentions the use 7812 and 7912 regulators. These regulators need at least a bit more (!) than 12V to do their job (that is: regulating the DC power to keep it around +12V and -12V).
I think that using a 2x12V (this is the same as a 24V center tapped) transformer will not give you enough potential for a stable PSU. I’d rather go for a 2x15V (30V center tapped). If you have a hard time finding this one you could also use a 2x18V transformer but this will result in much more heat on your regulators.
I would also recommend to buy a transformer that can deliver a bit more current than you actually need. This improves the performance of the transfomer.
Greetings,
Rutger
ps. I found this site to be very usefull in PSU design issues:
12vac transformers deliver more than enough to drive 7812’s, because the voltage gets multiplied by 1,4 when rectified. I made a +12 / -12 regulated PSU for midibox, just look it up in my WIKI, any and all comments are welcome (in the thread linked in the wiki article please, let’s keep the topics clean 
).