if you really want controlled round feed claw extractor + 3 position safety there's only a handful of options that I'm aware of, it'll be more of a hassle for the custom and you gotta want it.
Options I'm aware of from when I was looking to custom build CRF: Defiance, Winchester Model 70, Ruger M77, Kimber 84M/84L/8400, Montana 1999, Montana 2022
I don't believe any CRF guns accept pre-fits as all require an extractor cut (with the potential exception of the new montana 2022 but they haven't made any lefties yet?) Seems to put firmly out of the DIY barrel swap to me...
All but defiance would require buying a donor gun. Probably looking for $600-1200 for a donor rifle in one of those which is a fair bit of savings over the $3,000 for defiance.
When I was looking Ruger is by far the most available in left handed models followed by Winchester, then Montana 1999, and Kimber is most likely in last. New montana 2022 no lefties so far.
Weight-wise Dunno about defiance but winchester and ruger are heavy-ish and I think you'd need to try hard to get under 8lb all-in usually looking at 7-7.5# without a scope/rings. Kimbers are pretty light. Montana are about the same as winchester.
Defiance is the only one with a Rem700 footprint so you will have much better selection of stocks, rings, magazines, etc.
Winchester is probably the next easiest to find a stock for, mcmillan inlets a couple models as does stocky's and hs precision. Montana 1999 I've heard is a winchester clone, I don't know if that extends to inletting and bottom metal.
there's a handful of stocks available for rugers but no particularly light ones. HS precision was about the only composite option I saw when I was looking.
Kimbers are a round bodied action but not a 700 clone. Almost no one inlets for them that I've found except for a couple small makers: pendleton composites, MPI rifle stocks, bansner?
Most kimbers came with a blind magazine for a long time so most used rifles don't come with bottom metal, kimber doesn't sell them, and I never found any aftermarket options when I was looking. Would be a bit of a unicorn to find a LH model with a floorplate. Kimber would sell you a stock once upon a time but you gotta send em your rifle for them to install it.
I just had a custom done on a Winchester model 70. Ended up going with that because the action fit and finish was much nicer than my Ruger and stock options were a bit nicer. I wasn't willing to spend the $$ for a defiance just yet, maybe someday.
