The DR-DOS/OpenDOS Enhancement Project

Well, part IV of the licence agreement distinguishes between "the
Software" and "derivative works", though one would have to be a
lawyer to understand what exactly this means for users of such
derivative works. However, since the licence text does not specify
the length of the evaluation period for non-commercial use, only
that it has to be "reasonable", you can safely use it for any length
of time that you deem reasonable. Even if you would suspect that you
have already spent a reasonable time evaluating it, this would be
reset by switching to another version which you again would have to
evaluate.

Also, please note that even if you are bound by the licence terms,
the part of the licence that requires you to buy the software cannot
apply to derivative works licenced for non-commercial purposes,
simply because it is not allowed to sell them without a separate
licence.

Whether the licence requires you to buy the original software if you
only use a derivative work is not clear, I cannot find anything
related to that in the licence text. It probably applies to those
who compile Enhanced DR-DOS themselves, since they have to use the
original source code, but probably not to those who only use the
precompiled versions.