It might be OK, but you need to watch the clearance underneath the callipers. Standard race ones are not so likely to fit.
You may need deep drop callipers to get the space for the fattish tyre and mudguards. Going too close is a definitely bad idea, especially on th efront as a stick can get swept up by the tyre then jam the front wheel. It's not fun, especially the hospital food.
The callipers to look out for a RX100 deep drops. Alhonga also do some, usually a fair few around.
If you are going for new, then try a non-series Shimano deep drop calliper. It's a special for Audax bikes that Shimano make. Bike shops who know their stuff will be able to source it. Alternatively try an Audax bike specialist like www.spacycles.co.uk
Lots of Road shops will turn up their noses and say they don't exist, but they do.
As far as I remember the Pauls ones need special Mafac type braze-ons. It's probably not worth getting these fitted to the Horizon frame - it's not a bad frame, but I'd be saving my money for a Super Galaxy one or similar in 531ST.
Ride it or milk it!