should I look to get the offset rim for front/rear or the non-offset for the front and offset for the rear?
100mm disc hubs usually have more than enough dish to justify offset rims, although they will still be fine without offset.
Offset seems to be the only options available to me at the moment at Spa Cycles where I am likely to order them from.
Offset seems to be the only option
That's fine, Pro4 fronts have yards of dish so an offset rim is the first choice.
@swmlon started
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100mm disc hubs usually have more than enough dish to justify offset rims, although they will still be fine without offset.