The higher end shorts have more padding that is shaped to offer more padding where you need it most .
Decathlon shorts brand are btwin and yes the rating is the amount of padding you get 3 , 5 or 7
the 3 rated are £8
the 5 are £17 or £20 if you go for bib shorts that have straps over the shoulders( bit of a faff at a toilet stop more comfy as you get nothing digging into the waist )
http://www.decathlon.co.uk/cycling-shorts-5-flat-id_8165635.html
MTB slick tyres take less energy to roll and will make you a bit faster , i used scwable city jets for my MTB and they are great, gettiing them up to the right pressure helps as well so a decent pump helps as i used to run my MTB at 75 psi ,dependant on the tyres rating.
decathlon have these ...
http://www.decathlon.co.uk/26x15-randonneur-id_8026694.html