In my opinion, all the low pressure mixer showers available pretty much work, but you do get what you pay for.
Advise the customer of this, whatever your labour charge is, tell them to add £100 for a basic shower that may last a good few years and also may be a bit temperamental, or they can spend an extra £300 for a shower with a good guarantee that will give them years of reliable service.
It's pointless trying to guess what they want, I've come a cropper many times trying to give the customer the lowest quote/price, so have gone for budget items, many times ending up fitting something that was a pain to get watertight, or to balance etc.
If they are prepared to pay for a decent one, you can't really go wrong with the known makes like Mira, Gainsborough/Aqualisa etc.
Put the ball in their court mate.