Since 128-bit WEP is standards-based, there will be no trouble as long as the Psion APs are WiFi-compliant, which they should be. Roaming will work fine as long as the SSIDs and the WEP keys are the same on all APs.
You might want to advise your customer to consider upgrading to WPA2-PSK, which is very similar to WEP but far more secure. He might be unable to upgrade if he has legacy clients that aren't capable of WPA, but if there's nothing holding him back then I would definitely consider moving that direction. That's assuming the Psion APs are new enough that they can handle it.
Sorry for being confusing. As long as you're using the same security on an SSID on each AP, your roaming will work fine. WiFi includes standards-based security which will allow roaming to occur between different vendor equipment.
So WEP128, WPA-TKIP, and WPA-AES will all work fine as long as each AP is configured the same.