It's funny how many people read the OP and then disregard it. First of all, it seems to me that you are a Lefty, as am I. The struggle is real. So good luck finding a suitable lefty donor rifle at your average gun show, especially for a fair price. Right-handers forget about that. It's natural. Not their fault. Then you'd have to pay to get it tried. I think your idea of buying a tried action is sound, if you can find LH. Or you might look at a custom. I like Stiller Actions, because he always seems to have LH actions in stock. I think your biggest hurdle, as a lefty wanting to build a lightweight rifle, is acquiring a LH lightweight stock. Having recently traveled down the path you are seeking, perhaps the following will help. I found a used LH lightweight 300WM custom built on a Rem 700 action. It's a BDL, but other than that, it should be very similar to what you're looking for. Here's the build sheet for it:
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Here's what it weighed without optic, but with a 2-piece pic base.
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Real life weight of 30mm Talley extended medium rings. And weight with those rings and Zeiss Conquest 4.5-14x44 Scope (supposedly 17oz from specs)
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I the barrel shortened 4" and put a Leupold VX-6 3-18x44 on it for good.
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It's now just a tad over 8 lbs unloaded. Btw, I think my stock weighs 26oz, but I can't find a picture I took when I weighed it. The last thing I'll say is this. Some people say that lightweight rifles are more difficult to shoot accurately. I have found that to be true. Set a performance goal and do what you need to reach it. But going uber light is pricey and may induce some unwanted characteristics. A typical bottle of water weighs 1lb plus. You can shave a pound off your pack a lot cheaper and easier than you can shave that pound off your rifle. Consider that. Hopefully this helps. Good luck. Keep us posted on what you do.