@Retro_71
Brain Innovations will be selling legal JiffyDOS soon and possibly more. Individual Computers / Red's Retro will be selling the Cameleon (64 only) soon, one of the features is an accelerator.
I did my mod with a C128D motherboard as it is closer to the dimensions of an old school full AT motherboard and already has a built in 1571 controller. Even still it was too big and the case required modification. Get the biggest tower case you can find, especially in depth. Ton of work, the number of hours you will put in will be massive if you take this route. It was a must for me as I had to have 128 mode. If you go with just a C64 the task will be much easier, but no 128 mode.
Addons? Depends on what you want to accomplish. If doing a tower at minimum you will want a uIEC. The uIEC is astounding and dirt cheap. I will also be adding a 64nic+ (stand alone ethernet adapter) to the 128T, I have already pre-ordered one.
Any other questions, let me know. As you can probably tell, I love this project and love talking about it :-D