Welcome to the forum, we'll try to help in any way we can.
What is GVW of the 5th wheel? Older one's tend to be a little heavier than the newer ones.
What 1/2 ton do you have? What size engine?
Need more information to give you an educated answer....Need to know GVW of 5er, not empty weight, need to know what your trucks towing capacity is....You can take GVW of 5er and "ballpark" approx. 20% for pin weight you'll drop in the bed of your truck....add a couple hundred pounds for a 5th wheel hitch....
Sounds like it's do-able with only 21' 5er, but knowing weights is a must.
If you've looked at the 5er, I look for signs of water leaks around the ceiling and inside walls, look at hot water heater, stove, see if everything works to your liking....check the condition of the tires
Don't understand what you mean you would rarely tow it, if ever??? Why buy it then?
Let us know, we'll try to answer your questions.