First, you need to decide whether you will be keeping a landline (possibly dropping to a "measured", "low-use", or "lifeline" service). In an emergency, a landline will usually be the most reliable; VoIP won't work if your power, equipment, Internet connection or provider fails. However, if cellular coverage inside your home is good, you may decide that's an acceptable backup. If you have an alarm system without cellular backup, don't trust it to VoIP. Faxing is often iffy with VoIP, though you can also send faxes via an email-to-fax or web-to-fax service. Finally, if you have DSL, porting your landline away will cause it to be disconnected; you would have to get a dry-loop DSL installed, or get service from another ISP.
If you get service from VOIPo (or one of their competitors), they will supply you with a two-port ATA. Connected to two or more two-line phones, or to a two-line cordless base with multiple handsets, you will be able to make or answer a call from any phone, while another is in use. Porting your landline number (if desired) will be no hassle. You won't need to buy an OBi and you won't need to learn anything about VoIP.
If you are in the DIY mode, you can save more money and have a more flexible system. One choice is an OBi with a combination of GV and Anveo. Anveo's 911 is only $0.80/mo. If desired, you can get a "value" incoming number for $0.99/mo., though you'll pay for calls if you exceed the 40 free minutes per day. Outbound domestic calls are $0.01/min. Since GV will be your primary provider, I would expect your minute usage on Anveo to be very low. This system allows only one call at a time (except for three-way and call waiting). If you want two concurrent calls, you can add a second ATA. If the other ATA has a "router" function, e.g. SPA2102, you can set it up as a bridge, so it won't "burn" a port on your router. OBi will also soon be releasing an ATA with two Ethernet ports.