@SayeemAhmed 

If you want to buy X60, because you are not happy with performance of your Deco mesh, you should first figure why you are getting message "Abnormal WAN port speed." This message says your Main Deco negotiated just 100Mbps speed over Ethernet cable to ISP modem/router. Instead of standard gigabit speed.

If you manage to fix the problem (usually, replacing cable does that) and your Internet speed provided by ISP exceeds 100Mbps, you will see improvement with your Deco mesh.

Just replacing Main Deco X50 with X60, which you can do, but not fixing Ethernet cable issue will not bring speed improvements you expect to see.

  @SayeemAhmed 

With Deco mesh it is more important what features Deco unit supports than hardware version. The best way for you to test would be to borrow X60 from your friend, add it to your Deco mesh as Main Deco and see if all features you do use still available in Deco app. 

As for this specific hardware V2, it is quite old. Yet, don't expect newer hardware (V3 or V4) be necessarily better and faster: because of chip shortages during COVID and getting whatever substitutes are available, and because of inflation that requires cost management for components it might be possible that older X60 V2 is more powerful than newer X60 V3 and V4.

I am only speculating here, TP-Link does not offer full set of hardware specs and their performance benchmarks, but we are living in a time where manufacturers may degrade newer networking gear performance to hit cost, availability or power consumption goals.

To summarize, I would be more worried that this X60 unit is functional and that it provides all features you currently use. If you tested and it does, and your friend offers X60 with good discount, I would go for it if I were in your place.

  @SayeemAhmed 

This may be a simple question, but I am not sure what exactly you are asking.

If your Main Deco is "regular" X50 and not X50 Pro, its Ethernet ports are gigabit. When you see "Abnormal WAN Port Speed" message in Deco app, for X50 it means it will get just 100 Mbps from ISP modem/router. This is the fastest speed Main Deco can provide to devices connected to it, and if you have more than one device actively accessing Internet simultaneously, they will have to share that 100Mbps.

Main Deco X50 could deliver gigabit WiFi and Internet speeds, if it has gigabit connection to ISP modem/router and you subscribed to gigabit Internet package from your ISP.

Satellite Deco will deliver half of the speed of Main Deco, using wireless backhaul, at most. Satellite Deco M4 is WiFi5, which means it'll deliver less than half. Satellite X10 could deliver up to half of what Main Deco can.

If Satellite Deco uses Ethernet backhaul, it can provide same WiFi speed as Main Deco, with consideration of its WiFi protocol. Satellite M4 on Ethernet backhaul can provide up to 500-600Mbps, Satellite X10 - 800-900Mbps.