I drive an 86 syncro westy and I want to add front AC to my existing
rear AC. My current (seized and leaking) compressor is a SD510.

Bus Depot lists the SD508, the SD510, and SD709 for the '86 model
year, all are the same price. Am I correct in assuming that the 5
series compressors are 5 cylinder and the 7 series are 7 cylinder
(hence the 7 series would be more powerful. I assume I would just have
to change the fittings. Is there another difference? Are they
interchangable?

Since I am adding the BEHR unit up front I thought it might be prudent
to upgrade to the SD709 along with the upgraded 253 260 403 two fin
section condensor (versus the 255 272 413A condensor which is single
fins). Again I am wondering if the condensor is interchangable.

Lastly is the radiator fan. Volks cafe lists a fan assembly
251.959.455M at $445.00
which seems a little steep. Would I have to buy this whole setup (plus
the condensor) if I wanted to upgrade the fan?

Any advice would be appreciated...