Welcome! They are great bikes, but I warn, as HiDesert did, be careful with having the experience on an SV650 and going to a 955cc three cylinder. I say that not to presume you cannot handle the difference, but the weight, hp/torque, and size of the bike will surprise you at first. I went from a 1992 Yamaha Seca II 600cc (put 17k miles on it) to a 2000 Triumph TT600 600cc inline four, put 7k miles on it) to an '02 CE Daytona 955i (have put 12k miles on it) and now have another TT600. The jump from the Yamaha to the TT600 was huge, and the TT600 to 955i was not as large, but very evident. It sounds like you keep your head when riding, and that you at least have the MSF knowledge, so good luck and take it slow as you get used to it.
Best place to find parts is eBay (usa and UK), bikebandit.com, or the TRAt.net classifieds.
I bought my '02 Daytona wrecked and restored it into a Speed Triple clone. Check out my signature thread for more info on that. Yes, a 2002-2004 Speed Triple upper triple clamp will fit on a 2002-2006 Daytona 955i. Aside from the obvious need for handlebar and risers, you will need to get a 02-04 S3 clutch cable which is longer, and the half-clamps for the clutch and brake perches, and the master cylinder reservoir bracket if you use a stock bar. Daytona clamps have no threaded bosses for the mirrors, while the S3 clamps do. Here's a money/time saver: I found that a 2007 Suzuki SV650 master cylinder mounting/mirror clamp is a direct fit to the Daytona/S3 perches (along with all other Triumph models). Triumph wants $23 for the part while Suzuki wants $13. Also, the SV650 clamp is probably easier to find used. The part number is: 59671-32F00. You can use pre 2002 S3 mirrors or Post 2002 S3 mirrors (different designs, but same thread.) I have 2000 S3 mirrors on my TT600 converted to a Speed Four, and 2006 S3 mirrors on the Daytona 955i. There is a member selling a set of S3 mirrors in the classifieds I believe. A lot of info, but I hope it helps.
So are you going to buy the motorcycle?