UCG-Ultra... it has a reputation of being underpowered
There are complaints that UCG Max is going hot, too hot.Really not enough information here to offer good advice. What are you hoping to get from switching to UniFi? Easier configuration/management? Better monitoring? Better wifi performance? Depending on your goals, it might make more sense to replace the wifi end of things first and keep your existing router/firewall arrangements for awhile. But either way will not give you the single point of management that is one of UniFi's main selling points: you'll be managing the UniFi and not-UniFi parts of the network separately.
I will say that the UCG-Fiber is a poor choice if your ISP connection is not 5GbE or faster. You're paying for expensive high-speed ports that will be pretty useless without that. As @Tech9 said, the UCG-Max is more sensible for most people. (I don't recommend dropping down to the UCG-Ultra, though; it has a reputation of being underpowered.)

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