bike computer
it depends if you need gps , you can get basic non gps devices with wheel magnet that can track your distance/mileage but obviously you cant analyse or save the data like a dedicated device.
Plenty of apps for mobile phones to act as a bike computer , the only downside is that they tend to eat battery life
a dedicated bike gps device will generally have better battery life and the higher models will have real time maps and you are able to analyse your ride via apps from the manufacturer or 3rd party apps like strava.You can pick up second hand devices but be aware some of the older models do not have bluetooth so the only way to upload is via a direct connection to a computer via USB .
Anything in the last 5 years (?) should have bluetooth to connect to your phone .
the decathlon one linked on market place is not listed as for sale anymore so be sure its ok as i doubt there would be much support if you have issues .
Helmet
as mentioned above planet x do cheap lids , i use one for commuting and its fine but unless your sure of your size then decathlon is a better bet as you can try before you buy and their returns policy is brilliant .
Lights
you need to decide whether you are on goign to be riding in the dark and whether it will be roads with street lights or unlit lanes .I cureently run 2x 600 lumen side by side and thats just about enough for unlit lanes at night but overkill for the lit roads.
