3' is an absolute minimumfor aisle width.4' is better and will provide room for a viewer or another operator.
I am going to reccomend that as you work on your new plan, work on it as a Modular Plan.
My reasoning is this. Building this RR one module at a time will give you a little experiance in everything, it will give you the experience you need t be able to complete a RR the size which you are contemplating.
If you feel the need to run some trains,throw a temporary loop on a piece of plywood and let it run while you work on one module.
By the time you complete several modules you will see that the experience you are gaining is paying off and you may even want to go back and redo the first module.
Buying all the lumber you need to build the benchwork now is fine. It will give it not only a chance to acclimate but also dry out over time making it more stable. Either that or you could make all your dimension lumber out of 3/4" plywood rather then 1x3's as by its very nature it is more stable(less suseptible to climate changes etc.)
Try to focus on your mainline first. Getting the mainline right is all important.Passing tracks,Industrial Sidings etc can then be fit in afterwards.
Think Hidden staging!!!
With this your RR can become like theatre and trains can run the circuit,dissapear and another take its place.(perhaps even heading in the other direction)
Designing a plausible freelance RR is the most difficult thing to pull off but it sounds as if you are looking less towards realism and more towards fantasy so this becomes less of a problem.
While you are after a loop, try to think of the railroad as a point to point going from perhaps 1 division point to another(both being off layout-hidden staging)
Take a look at your favorite area of the country, look at where the railroad goes and how it operates.Change the name of the RR and the towns but use the reall RR to help you design.It will make life much easier.
I hope you find this little treatise helpful
