I have two Chromega D5 Supers in my darkroom that I print with, another in the shop I use for parts. It's the enlarger I learned on so when I retired and built my darkroom that's what I used. But as I accumulated equipment over the years people gave me lots of things, an Omega D2 for example.
As I gradually collected equipment negative carriers was one of the most interesting areas to deal with.
Here's something that might help you save some money. Omega D2 enlargers are older and more readily available/cheaper. And many parts are interchangeable with D5 parts. The negative carriers for D2's are the same size and shape as the negative carriers for the D5 with a slight difference.
There is a plate that fastensns the top of your bellows to your enlarger. This plate is also the surface your negative carrier rests on. The plate on a D2 has pins that engage with the negative carrier to keep the negative centered above the lens. The D5 plate does not have these pins, but instead relies on a piece of metal screwed to each negative carrier to keep the negative centered. Other than that, and position of some mounting holes, the plates are the same on each enlarger. They are interchangeable if you can drill holes/tap threads.
Retrofit your D5 with a plate from a D2. This will allow you to use D2 negative carriers, which are more readily available, OR D5 negative carriers for more modern sized negatives - like 645. .
But I will also say that you can print 645 negatives with 6x6 negative carriers no problem.
I have no idea what a speed carrier is.
If you get stuck, pm me and maybe we can talk on the phone. I'd be glad to help anyway I can.