The biggest difference between the '93 that came with R134a and any earlier 240 w/R12 is the size of the evaporator, so you would need to yank the airbox from the '93 and put it in the '90. One other difference I believe the '93 used larger tubing/hoses, and those ahould be easy to swap.
In '91-92 there is yet another larger evaporator, which will fit into the older airbox. Either way, it's a lot of work.
The PF condenser sounds like a good idea, probably the least expensive/level of difficulty to get better A/C.
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'89 245 sportwagon, destroyed by hit & run driver, RIP. '04 V70 2.5 T Sportwagon, 12k mi and '91 245 5-speed, 209k mi, replaced the '89