Lotsa questions ...
Is it worth saving? Well, it is a relatively common model, no great antiquarian or collector value. But if it is in good working condition, it is still perfectly competent for alignment of household AM and shortwave radios, and many older 'boatanchor' communications receivers. So it is well worthwhile for somebody who needs a signal generator for those purposes.
What's it worth? There is no set, going price for instruments like this; depends on who you ask. Some people like the engine-turned front panels on this series of instruments and consider them more desirable than the "plain Jane" versions. Others just see an ancient piece of test equipment. Probably the best thing to do is keep an eye on eBay auctions (not the "buy it now" prices, which are often unrealistic) to get some idea of what people are willing to pay in an online venue.
Where to get test cables? They can be fabricated easily. All the connectors and materials are still obtainable from larger electronics dealers.
Where to get a manual? Here's one source of a free download: http://www.ko4bb.com/manuals/index.php?dir=09)_Misc_Test_Equipment/Hickok
Note that the Hickok 177X and 188X are essentially the same instrument, except for the cosmetics.
What about those caps? Most modern plastic film capacitors (such as Orange Drops) do not have "outer foil" markings like some old paper capacitors did. Due to the way most modern film caps are manufactured, it no longer makes a difference which way they are installed. However, you still have to observe the correct polarity when replacing electrolytic capacitors.