For the first few weeks, don't dose ferts. Plants get stunted when transplanted, so they won't be soaking up nutrients very quickly at first.
Water from another fish tank isn't helpful..the bacteria for the nitrogen cycle live on surfaces...not free-floating in the water. So decorations, gravel, filter sponges, etc...from an existing tank will help speed your cycle.
If you cram a lot of plants in from the beginning, and get CO2 in there, its very likely you'll 'skip' the normal cycle. Plants will soak up excess ammonia, and the bacteria transferred from your existing gravel/decorations will further help. However, you do still have to treat it like a normal cycle, and test ammonia, nitrites and nitrates. If nitrates appear and start climbing every time you test, you can feel confident that the tank is 'ready'.
I'd set it up without fish, but add fish food like you were normally feeding. After a week, if you have some nitrates, I'd start adding fish, slowly.
And with a 120watt fixture over a 40g UK tank, you'll definitely want CO2 injection...pressurized is preferred, but DIY is possible.