I could go the higher pressure pump and 100 l tank, then later on change over to the VSD controller ?
If a 30/50 pressure range is ok, then the AL-KO pump is ok as a larger pump would be 'wasted'. The AL-KO pump is a jet pump and it sacrifices some pumping performance for better suction.
The Davey 60 L pressure tank (below) is a good price ($210).
http://www.justwaterpumps.com.au/davey- ... ssure-tank
The above combination is your cheapest option (apart from having a pump only) and you can always add another tank later if needed. You will need to sort out the 30/50 controller...manual or automatic? If automatic, will you also need an adapter and an electrician to wire the controller to the pump?
Your next option is a 40/60 psi pressure switch, pressure tank and a bigger pump that dead heads at about 70 psi.
The closer the pressure tank is to the house connection, the less friction loss but keep in mind where the shower is.
The next option is a VSD but unless you want an emergency pressurised water supply if the power goes out, you only need a small pressure tank. This then goes back to corporate_misfit's first post that talks of the savings by not needing a large tank and having those savings contribute to the additional cost of a VSD.
Time to start a spread sheet?