The PRV (Peugoet/Renault/Volvo) 2.8 litre V6 had its problems. Poor care brought out the worst in the beast. By 1988-89 many of the bugs were cured and was almost a good engine. Volvo dropped it by 1991, but it powered many 1990 and earlier 760s and 780s. You can probably find a good one of the later ones cheap considering nobody wants them because the early ones were so troublesome.
Look for an engine that "looks" maintained...clean oil filler, dipstick, air intake (throttle body), dry outside.

Changing to the turbo 4 cyl or any other engine is probably more labour and parts than the car is worth.