You didn't mention how many screens you have in your panel. The good news that you can share serial ports across the buss, so you aren't limited to four ports if you have more than one screen.
If you only have one screen, you don't have a choice with the VP-X, 429, and transponder. The first two won't work with a connection to the EFIS. The transponder needs the encoder output from the EFIS or a dedicated encoder. Fortunately, most of the functionality of the 650 can be used through the 429. But I would still hook up the 650 over the SL-30. I don't recall off the top of my head, what features of the 650 are 429 based versus 232 based. A good read of the install manual will help you answer that question.