Here are some things to consider when deciding which UniFi product you should choose to replace your current equipment.
Routers
Do you need SFP+ for your switches or ISP connection?
Do you need IPS enabled? If yes, what throughput speed do you need with it enabled?
Do you need VPN for site to site or remote users on the router?
These questions will help you narrow down which UniFi router you should purchase or whether UniFi is a fit at all for your requirements.
Switches
How many ports do you need?
How many SFP+ do you need?
How much PoE budget do you need?
These questions will help you determine which line of UniFi switches will be needed to meet your requirements.
Access Points
Indoor or outdoor?
Form factor?
Client density per AP?
Typically you'll want to go with the flagship, latest generation UniFi AP. But these questions will help you determine if you need an outdoor rated AP, a particular form factor AP for example table top or in wall, or if a higher density AP is needed.