I don't have any suggestions regarding the stitch color. However, if a contrasting stitch color appeals to you, I encourage you to go for it. I've seen quite a few Porsches with contrasting or "deviated" stitching and it adds a nice detail to the car.
Your idea for adding the seat heating is a good one. I used the instructions in this link
https://www.dennisvogel.com/heatedseats/ to retrofit mine. As Dennis Vogel says, "These instructions assume that you'd like to upgrade to a two-stage heating system for driver and passenger seats that is as close to the original P14 factory option specifications as possible."
The project did appear daunting to me. I'm not the world's greatest DIY mechanic. If you have decent skills and use patience, you should be able to achieve great results. You won't be following his instructions exactly if you use aftermarket heating pads, but you should be able to adapt to the differences quite easily.
Please note that most if not all the components are available used. If you buy them new, it will be expensive. I'm currently attempting to retrofit some 987/997 adaptive sports seats in my Boxster. I didn't want to remove components from my current seats, so I needed some wiring harnesses and relays. I found the seat wiring harnesses for $16 and the relays (part number 996.618.521.00) for $40 at DC Automotive. I think a new relay would cost over $100 at a dealer.
Good luck with your project.