to be honest jankers its a case of how long is a piece of string, i too primarily run RC marine models but have dabbled in trains now for about 2 years or so...
I now model N gauge, my layouts are normally built on hollow core doors ( lightweight yet rigid )
so just over 2 feet wide but about 6 and a half feet plus in length....
if you want a simple double oval, and a basic tank loco or two and a few trucks then i expect you would be looking at a cost in the region of £200 all in..... and a graham farish starter set may be a good starting point, comes with a tank loco about 6 wagons, basic oval and power transformer- all this for about £60 - 80 quid for the smaller freight style starter sets, but if you want all singing all dancing electrified points, scenry, DCC locos or something like KATO rolling stock then the price tag could rise dramatically- so its depends on what you buy....
i dont know if you use ebay at all but this may be a good place to look at second hand values of rolling stock, also check out some of the stores like rails in sheffield, they often sell used goods but they also sell new items, they sometimes list track starter packs and various grades of used rolling stock..... its worth looking to establish current prices for N gauge stock.
also it may be worth picking up a copy of say railway modeller magazine- listed in the back will be shows and swapmeets and collectors fairs, this may also lead to a good starting point for purchasing a good start up bundle !
anyway good luck and keep us posted on progress ! - and when it comes to buying tunnel mouths, bridges and scenic stuff for your N scale layout, let me know- i sell / distirbute alot of this stuff and also sell it on ebay....