Very little difference between the 105 and 605, both are 10/100 switches. The 605 is the newer model though.
Your frient is correct the DG814 will not work with NTL. You need a broadband cable modem such as the D-Link DCM-200 with co-axial input. I don't know if NTL have their own?
My personal advise is do not go to NTL. I am with BT ADSL; it may cost a few pounds extra but to me well worth it. You can then use a combined modem/router/switch such as D-Link DSL-504. Typically click here
As for the printer option, this gets a little more complicated.
1)You can run your printer off a dedicated PC (via USB or parallel port)and then other PCs on your net can access the printer through your net to the main PC. Disadvantage is that your main PC must be on for others to access the printer.
2) You can buy a printer server that plugs directly into your printer and is wired back to your router or switch.
3) (Best) you buy a combined router/switch with internal printer server, such as theD-Link-704
The above options are also available as WiFi units.
Trust this helps a little bit and post back if you require any clarifications.