@grantly
I have just finished a power supply and TPA3255 EVM system, after much 3116 tinkering inspired by this excellent thread, and for what it's worth:
1) The 3255 EVM board is relatively big as mentioned, and also tall because of the huge caps mounted on the underside which you tend not to see in the pictures.
2) If you can get an EVM board for half price it's certainly worth the money, partly because the board exposes all the functions of the chip, even the ones you will never use. This also makes it quite complex, covered in jumpers, and you have to navigate the many different schemes for connecting external bits depending on the jumper settings. The board itself is a marvel of engineering, quite lovely, the construction quality and component selection are above reproach as far as I can see, and I presume that whoever designed it was also sitting next to the person who designed the chip.
3) If it comes to paying full price for an EVM board on a tight budget, I think it's still worth considering the 3e-audio stereo 3255/325X boards. I did investigate them, and when I contacted them via the bay to ask tech questions they even offered to customise the board for me. But that only applies in the normal case where you only want a stereo setup. That makes the board much simpler to hook up and configure, and it's also much more compact than the EVM board. I decided to avoid the very cheapest boards just because ill-considered cost cutting and wonky engineering seemed like a possibility, and they're not that much cheaper.
4) Maybe also have a look at 3250 and 3251 boards, just because they are easier and cheaper to power and won't require any thought about heat generation. The 3255 chip is powerful at up to about 600W, and I tell myself that it's always possible to turn the volume down, but 95% of the time I would have been fine with a 3251 and only in party mode is the big power needed. In any case you probably wont need more than about a 400W 48V switched mode supply. You can get boards or finished units.
5) If you case it yourself, you'll also need many tools and appropriate cabling and case mountings so on, an AVO meter, and this can add a lot of cost if you don't already own them. I didn't really think about this properly which is why I mention something so obvious.
Good luck.