Just a word of warning, The PAL version of beatmania has gotten pretty negative thoughts from beatmania players. I would go with buying a US bundle, maybe two if you want to controllers (you can find them for cheap if you can find a place that still sells it, However, I dont think you live in the U.S seeing you were on a uk website, so either a U.S. bundle, or Japanese controller would work).
next you are going to have to worry about playing japanese games on your PS2. You will need swap magic, or some other type of swap disc that will allow you to play imports.
you were also talking about the buttons on a beatmania machine. Well if you find those parts they are VERY expensive if I remember correctly. they are called Sanwa buttons, and I heard they cost $40 for one button, so getting a normal controller will do you a lot better, as stated before.
Hope I helped a bit.
Plus trust me, you wont want the old 5 key games, you will want IIDX, the newer games that have 7 keys and a lot more music.
You can find most of the games on playasia...
Buy Video Games for Consoles and PC - From Japan, Korea and other Regions! - Play-Asia.com
A word of warning, and I am not sure if you have read this correctly but you WILL NEED a swap disc (swap magic), or a japanese PS2.