Hi. As you seem to understand, getting the track right is everything. All the pretty scenery and little people will be wonderful, but a model railroad it will be in visual terms only. So, the first item is to have a meaningful track plan. What sort of layout theme will it be, and what length of locomotive and rolling stock do you anticipate runnning on it over the foreseeable future? Think in terms of upscaling, maybe to larger steamers if you ever get a hankering for one. What track plan will allow you some interesting train moves rather that roundy-round?
Next, are you going to have grades and eased curves? Grades need easements, too. How much room will these take and still allow you the length of rise and fall into/out of grades? You will need a minimum of 3"...absolute...for over-rail clearance above the lower tracks. Don't forget that above that minimum clearance must be something, a bridge, an overpass, whatever, but that adds to your real grade.
The key to it all, once the planning is seriously considered and decided, is to lay roadbed that is as planar as you can get it. If you want a bit of super-elevation on the odd curve, you can shim up the outer tie ends with some cardstock slivers or some layers of masking tape, but what those shims or tape use for a footing is what the entire rail system must have...smooth and level (side-to-side) roadbed. If your tracks can dip or if they kink upward at any joins, particularly on curves, and certainly where you add a turnout, you can anticipate hair-pulling and nasty words, and it won't be from amused onlookers.
Lastly, prove your trackage before you do the final gluing into place with a cheap latex caulk spread butter thin. By proving, I mean run engines backwards and forwards, with and without trains, through all turnouts and around all curves. Cover the entire system before you "nail" it down and begin the scenery or ballasting and weathering of your tracks.
One final tip: don't have any tracks closer than 3" to the edge of your layout...5" would be safer still... but 3", especially with an outer barrier of trees or a berm of earth, maybe some rock faces or cuts, will prevent the inevitable tumbles of expensive items to the floor ...did you say 7'?...below.
