A couple of questions Junctionmad.
Will the rails need to be all bullhead or a mix of bullhead and flat bottom rail.
Are you going to use any ply and rivet sleepers in the build.
Do you want exact representations of the crossings and switches, ie ABCD size 1-6 1-7 and so on.
C&L point kits are a good place to start when wanting to put them together without any of the filing of track.
Exactoscale point kits are on another level altogether and come with all the chairs, slide chairs, bridge chairs, and the special chairs for the crossings and vee's. Personally if you want the track to look right in appearance then go for the Exactoscale kits.
Exactoscale and C&L also do sleeper bases of the concrete, and wooden type.
The Exactoscale wooden sleeper bases have little pimples on the sleepers where the chairs sit on them as the chairs all have holes on the underside. Even with this system if you are thinking of using one of the wider gauges then get some track building gauges.
Fran is spot on about re wheeling and compensating anything wider then EM gauge.
The only advantage I could see you being able to achieve in 00 gauge is being able to use bullhead rail and having the correct sleeper spacing. If you are happy with using all flatbottom rail then Pete Harvey produces a nice tool for spacing the sleepers correctly in Peco track.
Whatever the choice you make good luck.
Rich,