There are two well established brands currently producing models for UK N gauge, 'Farish by Bachmann' and 'Dapol', and occasional productions from others. Dapol have produced some coaches based on the LNER's well known designs by Nigel Gresley, but what the current availability is like I really don't know, suplies of model railway product have been patchy these past two years; and there's probably some s/h product from various sources, including the old Graham Farish outfit which dominated UK N gauge for many years. Best place to start for a realisitic overview probably dealers in your area: here's a useful site enabling a check of what's local to you.
My principle interest is GNR, LNER and its subsequent operation under BR, using OO. This subject is much easier in the BR period from the mid-fifties as there is so much more choice in ready to run rolling stock, and I believe much the same applies to N gauge. That means coaches in BR livery, and many of BR design, rather than the varnished teak of the GNR and LNER Howlden, Gresley and Thompson vehicles; that's 'going with what is available' as I don't plan to build much myself by kits or from scratch.
Of course there are heroic types who are prepared to do it themselves, if you haven't seen it here's an N gauge layout by members of the Model Railway Club to marvel at: