Probably your best bet would be to find some way to try out line systems after your rod is complete.. maybe a dealer who has reels loaded with different lines.
Check Some of the dealers who sell a lot of spey equipment.
Otherwise a changeable head system might work.. you would have a running line and then could change out a skagit, scandi or custom made head that would be suitable for your rod and your casting preferences and would give you flexibility in differing conditions...winter, summer, etc.
Casting preferences usually come after you have had some casting experience with different rod and line combinations. Preferences are also determined by the kind of fishing circumstances you anticipate.. fishing deep, weighted flys, surface flys, size of river, etc.
Spey casting is simply more complicated than regular fly fishing because of the different approaches to casting and fishing conditions.