I have plans for two such vests, myself: one in 13-ounce wool, one in Marton Mills poly-viscose (11-ounce)---and possibly a 16 ounce later. I'm going to look at a couple of patterns for guidance, such as Folkwear #222, Laughing Moon #109 plus lots of photos here and some examples in my closet.

I will avoid lapels, on the grounds that matching them in any meaningful way to a bias-cut tartan would be beyond me! If I can get the tartan to look decent at the front, I'll be ecstatic.

Satin back or tartan? could go either way. A tartan back lends itself to being worn without a jacket, and I believe it would look dressier alone. Satin might be cooler, either with or without jacket.

In one case, this may be decided for me by the amount of tartan fabric available. I have about a meter of tartan to match my muted Wallace. That may not be enough for a bias-cut front and full back.