I'm like ADC on this one go low you can always come up...no guarantees with used guns. AND go with a synthetic stock IF it is to be a hunting gun.

There's also quite a few Mdl 24 parts, all that you listed, available on ebay. As mentioned, Numrich Arms is great source as well as Brownell's. You'll pay for it if they have it, but if you need it, you need it.
I think $150 sounds high, but I'm cheap. I'd offer $50-$75. Just my .02 cents.

I agree with what almost everyone else has said. Here's my take. How bad do you want it? If you want it, offer less-period. Not worth a whole heck of a lot. You can pick up single shot weapons for about the same price in much better condition if you are looking for the single shot weapon. If you were looking for a combo, once you have it, at your leisure, you can reconstruct it as pretty or as ugly as you are able to or need to.

I just bought the same gun a month ago, it also needed a trigger guard. I found mine on eBay for under 15 dollars free shipping. was not hard to find and not expensive. Problem is they are plastic. I have read that there was a company that made metal ones but was unable to find them.

I had one for a couple years. 22mag/20ga. 22mag IMO requires a scope. Gun had bases for clamp on 22 type not suitable for 20ga shooting. Couldn't find any possible use for the gun.

I would be carrying it on a snow machine for up to 16 hours a day, so it wouldn't have a scope on it. If you strap a scoped gun to a snow machine, the scope will get bounced off two or three inches in no time. I carry my .223 varmint rifle with a scope on it slung on my back so it don't get off sight. In my experience, you don't always have time to stop the snowmachine, turn it off, take off your goggles and gauntlets, get off and sink in waste deep snow, flounder to the back of the snow machine, unlatch a gun case, chamber a round, take the safety off, and still expect a wolf or other varmint to hang around. So I carry my .223 on my back, and some kind of shotgun strapped to the tunnel for quick access for rabbits, grouse, and ptarmigan. It cuts out the floundering to the back the machine and struggling with a gun case.