I have a regular tap in my toilet. I am thinking of replacing it with a toilet handshower or bidet. Do I have to worry anything about the inlet pressure for the hand shower to work?

Well of course you have to be concerned about the pressure. I am reminded of time I stuck a fire hose up my Rectum.

No, you should not worry about pressure. Retrofit bidet seats works on regular house pressure. Also, bidet seats have its own flow restrictors and aerators making water soft and bubbly.

Hope that helps

Milo