1.No real way to know if it has been rebuilt without opening it up. I don't know of any "industry standard" way of marking a rebuilt engine. Every machine shop seems to do it different, at least around here.
2. It depends on how the compression was checked, and how long the engine was sitting. With a good, strong battery, all of the plugs out, and the throttle plates held wide open, yes those are poor numbers. Try a shot (1 or 2 squirts from an oil can) of light oil in each cylinder, crank for 30 seconds, apply more oil, repeat a few times. If compression keeps improving, you may just have stuck rings from it sitting.
413 pistons are very expensive and can be hard to find. It might be much more economical to find a 440 to build if it comes to a rebuild. That long ram should fit a 440 just fine.
3. The missing valley pan should not be a big deal, IF it is not rusty under there. A 440 valley pan should fit. A 440 is basically a 413 with a bigger bore, and a few other minor differences.
Holley carbs are not correct for that application, I am fairly sure there should be 2 carter carbs on there. The wrong carbs will greatly diminish the value of the package, possibly by as much as half or more.
Now, for my opinions.
The 413 is a really cool engine that is not seen much anymore. That being said, because you don't see them anymore 413 specific parts are expensive. You can fit a 440 anywhere a 413 will. With the cost difference in parts, building a 440 over a 413 is like getting paid to have the extra 27cid.
Of all that you are looking at, the long ram would be the most valuable part, if it were all there and all correctly date coded. With the wrong carbs, probably wrong aircleaners, and wrong linkage probably, the only "valuable" part would be the manifolds themselves. Original exhaust manifolds would be worth something too.
The second most valuable part would be the big block PB727. These are getting really hard to find. In rebuildable condition, it could even be worth more then the bare manifolds.
The 413 itself would be hard to sell unless you found someone needing one with that date code.
For me it would really depend on price, and how bad I wanted a long ram.
You can check eBag for going prices on this stuff.