Ok well first neil is correct about ebay kits they are best avoided.

Problem with kits in general is that they are miss-matched and often your getting inferior something or other.
The CNC4YOU kit for instance has 36v PSU which is too low voltage for any decent speed from the motors supplied. Those motors will easily take 65-70V but the drives they supply are only rated 50V max. They are lower quality analog drives and so they supply 36v PSU because they don't want you pushing them too close to there Max voltage rating where they may blow up.

You'll find many suppliers doing this and this is why those who know and do there home work realise or get advised to buy Decent drives and if want to save money build your own PSU thats perfectly matched to your needs.

Cuttting corners on the electronics is THE most common mistake made with DIY CNC. The Heart of the machine is the Electronics and Cheap stuff just cripples any machine.
Steppers get there speed and To some degree torque from the voltage you put into them. They also produce electricity when de-accelerating. This means that you need drives with some overhead on there Max rating, typicly 10%.
Now to get good performance with 3Nm you really want to be running them over 50V with 65 -70Vdc being ideal voltage running on 75-80Vdc Max drives.
Running 50Vdc max drives will mean using 40-44Vdc max voltage which means the motors will be running roughly 30% below what there capable off.

Now people often build there own Toroidal PSU because correct size PSU's are hard to find or expensive. So building your own means they can get exactly the voltage and current they need at sensible money.! . . . Scary shit your probably thinking.!! But it's really not and very easy really when explained so don't be put off this option.

Honestly don't cut corners on the electroincs you 100% will regret it if you do.