To your first question, I'd check the general fit and finish first because that is very easy to do...parts all fit properly, wood to metal fit is good, no misaligned sights, bore is clean and no issues with rifling, muzzle is cut well with no dings or issues at the rifling, etc.
Then I would test the action for function with the .22 dummy rounds. Field strip it and look for any noticeable burrs or poorly machined parts. If the gun functions well with the .22 dummy rounds, fit and finish is good, and there are no visible problems seen when its field stripped, then go for it.
Now to the second question...
I have a Marlin 1894 CSS in 357 Magnum pre-Remington. Given my expected use for this rifle, which is NE Woods plinking and hunting, most shots within 50 to 100 yards, and my use of both 357 and 38 special ammo, I went with the FP series click adjustable Williams Peep Sights. They work great and the rifle is very easy to handle in the woods. There are also different apertures you can use for different conditions, which is great. There are also Skinner Peep sights and other manufacturers who produce a good product.
The 39a with Marlins micro groove barrel is very accurate. You could scope it. But that depends on your parameters for use. If you will be knocking around the woods with it, then it probably doesn't make sense.
Just one more point... If you go with a good set of Peep Sights, in most cases you will need a new front sight as well...