@Hawklord 

For your bulgalow, the answer is this: the best solution is 2-unit Deco M9 Plus set. There may be multiple options for garage and shed, but I'll need you share more information to see what would be the good solution for them. 

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With your Internet speed you do not need to spend money on WiFi6 hardware. WiFi6 can deliver faster speeds than WiFi5, but on shorter distance. WiFI6 is also more easily impacted by WiFi interference and noise, such as WiFi signal from your neighbours' networks and from household applicances.

Main Deco M9 Plus, which is WiFi5, can deliver over 600Mbps on Gigabit Internet, Satellite Deco M9 Plus can deliver up to 450Mbps. It is more than enough for your needs, even if you plan to upgrade your Internet speed in the future. As long as Internet speed is under 500Mbps, WiFi6 is not necessary.

I have Deco M9 Plus at my house, I got these speeds with my home network.

If your budget is tight and you do not plan to up your internet speed in the future, you can try 2-unit Deco M5. Depending on the size of your bungalow and distance between Main and Satellite Deco, they could be enough for your 150Mbps Internet link. Main Deco M5 should be able to deliver up to 450Mbps and Satellite - up to 200Mbps, which are my test results.

Whatever you choose, M9 Plus or M5, get them from the place with return policy. You won't need more than 2 weeks to figure if Deco works for you or not, but if it does not - better have an option to return Deco hardware for a refund.

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As for garage and shed, what do you have for them now? You already have wireless security cameras in both, right? How are they connected to your home network?

  @Alexandre. 

Thanks for your reply and you have confirmed some of my thoughts. The Deco M9 plus was top of my list until I came across the X20,X50,D9 etc etc.

My budget isn't tight but I do not want to waste money on performance and features that I will net use. Regarding the garage I currently have a Netgear range extender plugged in there which provides WiFi for the cameras in the shed and garage although the signal is weak and often breaks up. My thoughts are to get the M9 3 pack and put one of them inside the garage, would this work ?

If I find that a satellite M9 doesn't give s good enough signal in the garage I do have a TP-Link Powerline set which I could try instead. Would the primary unit need to be plugged into the ethernet port of my BT Hub or the M9 router or would it not matter.

  @Hawklord 

What they don't tell in WiFi mesh marketing materials, and that applies to any brand, not just Deco, is WiFi mesh nodes communicate with each other using standard WiFi protocols. Which means, if your Netgear range extender gets weak signal from the house now, Deco M9 will probably be the same and won't deliver better WiFi. Unless, of course, you can position your new Deco M9 set in the house in a way that delivers better signal to garage. You would not know until you try.

That also may not solve the issue with WiFi coverage in shed. There is one other thing to consider: TP-Link does not sell Deco M9 Plus in single units. You can buy 2-unit set and 3-units set.

If you want to have Deco nodes at both garage and shed, you may need to buy two 2-unit Deco M9 sets, total 4 units. You have to make that decision ahead. 

I would suggest to consider garage and shed separate "islands" from your house. Start with the house, get 2-unit Deco M9 plus set, configure it, become familiar with Deco mesh and that may help you to decide how to proceed further.

It is also possible you may have no other options but rely on powerline technology to have reliable WiFi in garage and shed. Deco M9 can piggyback on standard powerline adapters, but not every powerline adapter is compatible with Deco. If you already have powerline adapters at your home, you can check if they are compatible with Deco, when your Deco set arrives. I can give you details how to check compatibility.

You said "I do have a TP-Link Powerline set" - what is the model name of that set?

The rest of my recommendations will depend on path you are going to take:

1. Buy 2-unit Deco M9 Plus set for your house. Consider garage and shed separate "islands" from your house, explore different options you can have for their WiFi coverage. 

2. Buy 3-unit Deco M9 Plus set for your house+garage, and see if that works for you.

3. Buy two 2-unit Deco M9 Plus sets, total 4 units, to have Deco nodes in garage and in shed, and try different configurations to see if you can make that work.

  @Alexandre. 

Once again many thanks for your detailed reply.

As you say I won't really know what will work until I try it but I have decided to get the 3-unit Deco M9 plus pack first and if 2 units will cover the bungalow I can try the third unit in the garage.  The powerline kit I have is the TP-Link AV1000  TL-WPA7617. If I use this would I plug it into my BT home hub or the M9 router ?

  @Hawklord 

You can plug in house powerline kit adapter into BT Hub, into Main Deco M9 and even into in house Satellite Deco M9 if it is more conveniently located. All three options should work.

As long as you plan to keep BT home hub, I would recommend to configure Deco mesh to run in Access Point mode. By default, Deco mesh runs in Router mode with Main Deco playing the role of the router. You don't need second router in your house, you already have BT hub for that. Running Deco mesh in Access Point mode will make Deco integration into your home network much simpler. BT hub will keep managing home network (IP addresses allocation and such), Deco mesh will provide WiFi coverage.

You can switch Deco mesh to Access Point mode after you did initial Deco mesh setup and configuration. See How to set up Deco to work in Access Point mode

  @Alexandre. 

Now I am even more confused as I wasn't aware that the M9's could be configured either as a router or access point and all that I have read says that for the mesh system to work you switch off the WiFi router function in the BT home hub and replace it with a M9 router.

Wont setting the M9's as access points not make it act as a mesh network as walking from room to room with a mobile phone it would have to switch out of one access point and log into another rather using a common login SSID ?

I already have 2 access points in my bungalow but each one has a different name on the wireless network and that is the reason for wanting to change to a mesh setup.. 

  @Hawklord 

Worry not. This is just TP-Link Deco terminology. With Deco running in Access Point mode you will still have WiFi mesh functionality: common SSID, seamless mobile device switch between mesh nodes.

Just to give a final update to this thread.

I ended up getting the M9 3 pack and now get excellent WiFi signal all around the bungalow. The signal just about reached the camera in the garage but not the shed so I added an M3W extender in the garage which now gives very good signal to garage, shed and garden, so a very happy bunny.

Thanks again for all advice given.