crossed the path, ..... or will have when i know what width to cut HELP

ok guys thanks for your help with the path, decided im gonna cut out two sections for the track to run through , having not yet bought my digital starter set with two locos and a box of straights 1.2 metere (LGB) the set is
LGB78257
G Scale
MTS Digital Starter Set
i was wondering what clearance i should give it , or how big a gap i need to leave …also im planing a tunnel just a straight tunnel under my soon to be arranged rockery what size does the tunnel need to be height width for reasonable clearance … . also where can i find this starter set in the uk at a reasonable price , ive seen it available in the us for just over 350 pounds but cheapest in the uk is £561 pounds , can you get a convertor to use the us version on a uk power supply.
sorry to be a pain but i am a complete novice to garden railraoding
regards neil

as wide as you need for one track or the width X3 for 2 tracks , that will give you clearence for passing wide loads, [:D] Try EBAY ( but buyer beware [V])or try some of the links on the home page.[:)] as for as the tunnel your biggest, tallest,and longest car needs to able to pass thru it , Try building with a portal as the guide or 2x10’s side’s top and bottem as a way of preventing tunnel colapse