This particular Underwood 3-bank portable is from 1923-1929. The earlier ones only had the CAP key on the left side, but yours has the CAP key on both sides, which started in 1923. The serial number will give you the exact year, and the serial number reference lists can be found on the Underwood page of the Typewriter Database here.

As far the case key, that is usually missing from vintage typewriters. Having a case key is just considered a bonus. Even when I actually have a key, I save them but I never lock the case on the chance that a several decades-old lock will jam up. Leather handles are frequently missing. The cheapest fix is using something like a small leather dog collar. The more correct fix is to buy a new handle from a vintage trunk/suitcase/case parts seller and replace it, but with leather handles in the original style (mushroomed ends outside the brackets) it often involves removing the metal brackets to install the straps which can risk damaging the case or case hardware.

Price is going to vary. I would check SOLD listings on eBay for an idea of the approximate value, and use that as a general guide, understanding that condition is important, and that sellers who specialize in typewriters--especially fully restored ones--are able to sell machines for amounts well above the common price range. The typewriter's local classified value will be significantly less, and its antique store value will vary above and below, as well as differ regionally.