The Reims Rocket and Skylane were both OKish for proper touring.
Installing an HSI/RMI combination in a standard 172 does seem a little OTT, though. Have you considered the weight penalty?...those gyro units can be quite heavy in an aeroplane which doesn't have that much payload in the first place.
Having said that, if you make a lot of IFR approaches in the 172 HSI (for ILS and VOR) and, to a lesser extent RMI (for NDB) are a big, big safety benefit.
Any Avionics shop should be able to help. Paul at Tatenhill seems to be most people's favourite.
W