If its an old beater as you say, no reason for a new one.
Just make sure the used one has NO chips or blemishes
around the edge at all.
Installation of these isn't too hard.  I think you clean all the
old adhesive off as best you can, then put on primer for the
butyl tape, apply tape to windshield and using several friends
install it carefuly as you only get to set it in once.

I always go for the new windshield for the simple reason that you get
a super clear and warranted windshield vs. one with who knows how many
miles and roughage on it.

                    ...Ron

If you could inspect the used window meticulously, reject anything with
a single nick on an edge and install it yourself, a $50 window would be
worth the effort.
But if you are talking $100 for install plus $75 for an unknown and
unseen window, the price is too close to the new window price to take
the chance. On the other hand, if you could find someone to install the
used window and warranty it for at least 30 days, that would generally
be enough to be sure there were no latent defects. Many installers,
even for new glass, refuse to warranty anything with a vehicle that old
because of  rust. Some just won't warrant it for leaks.

Used.  Just make sure the windshield is not nicked from being taken out.