Why are you replacing all the hoses and the evaporator? Are they leaking? The only reason to replace static components would be if it is leaking.

Although I'm not a proponent of changing the 123 a/c systems to 134 unless you live near the artic circle, if you're going to do it, there is indeed a 134 expansion valve available. There is also a trinary switch available that will change the high and low cutout pressures to those needed for a 134 system.

Also along the lines of changing to 134 is the condensor. You need at least 20% more exchanging capacity to make up for the loss of capacity given up when changing to 134.

If you are starting with a system in reasonably good shape, it would be less costly and less trouble to simply stay with 12.

My $0.02,