opportunity to buy 2.3l w/ dual plug(135k mi.) w/ 5 spd. for $675.
Is the price reasonable and is it worth the effort to swap in to a '94 Ranger w/o TOO many issues?

 

Does the '94 have a 2.3l engine with manual trans in it now?
If not you will need to get the computer and wiring harness from a Ranger that did have a 2.3l w/ manual trans.
So it would not be an "easy swap".

Is '94 a 4x4?

$675 is a relative number.
Is engine/trans complete, i.e. all sensors and manifolds attached
135k miles is low, what year is the engine/trans?

A running pick up truck usually never drops below $500 because of it's utility value.
I would look at local "for sale" papers and websites to see what rangers with 2.3l w/ manuals are going for.
Might find a running truck with major body damage for under $675, that way you would get all the need parts for the swap.

In this world what something "looks like" has more value than how it works, lol, so lots of dings and dents or body rust lowers a cars valve more than a poor running engine, which is good for your project.