I am trying to figure out how to get the spindle going with my breakout board and running properly in Mach3.
The board is a KLDB25.
The spindle is a SunFar E300.

What is the best way to physically connect and also how to setup in Mach3?
Thanks!

You will have to reprogram some parameters in your VFD.
Here you can download the manual for VFD: http://stepdir.ru/komplektuyushie/dlya-frezernyh-stankov/invertory/invertor-sunfar-e300-2s0015l.html

Instead of using multifunction inputs you will have to set that the speed input is set with analog input.
On your BOB you have 0-10V output and according you have to wire it to your VFD.

Read the manual.

unfortunately I cannot find a manual for my BOB, but it looks as if every output has an opto-iso... can those run an analog signal for the spindle?
When you say it has a 0-10v output, is that a specific pin cause they all look like the same type

Unfortunately, your BOB does not have analog  output (PWM).
If you intend to use LPT port, do yu self a favour and buy one of these; https://www.aliexpress.com/item/32830611064.html?spm=a2g2w.productlist.3.1.30692b5cZiUzzE&sku_id=65015597255&gatewayAdapt=glo2rus
It as a relay to turn the spindle ON/OFF as well as analog output 0-10V

Is that one or a similar one available in the USA, preferably ebay?

Just search for "Mach3 5 axis .... "

I got this one...
https://www.ebay.com/itm/174725161345
looks like it converts 5vdc from the USB port for step/dir axis signals.... then it also has 12-24v input for the 0-10v spindle output.

Then I ordered one of these:
https://www.ebay.com/itm/175293058532?var=474511654200
Chose the 24v 2A model

Should be everything I need I hope... yes?