Hi -
it might not have been made clear that you can mix and match set-track and flexitrack. For example, Atlas (US prototype track) has both flexi-track and matching set-track in both Code 80 (taller rail profile) and Code 55 (closer to prototype). Peco has the same in UK-type track (UK in terms of sleeper placement and the look of the turnouts). The good thing about this is that one can use the set pieces in yard areas (so you don't have to cut and solder lots of flexitrack) leaving the flexitrack for long sweeping curves or straights. Atlas Code 55 set-track in N Scale has lots of different pieces of different lengths, curve diameters, turnouts, crossings etc etc. With set-track you don't have to solder if you're not comfortable with doing that (though connections are usually stronger if you do). It also allows you to re-arrange a first attempt at a track-plan if you're not happy with it. If you've cut flexitrack to a plan, they may not fit in a revised version.
I'd recommend starting with a set-track that also has flexitrack linked to it (e.g. Atlas or Peco). Bachmann EZTrack and Kato's Unitrack both suffer from the lack of expandability with flexitrack (I <think> Kato doesn't have flexitrack, but you could check that on their website).
I'd advise doing some price comparisons on the better N Scale sites (often much cheaper than list price). I'd recommend
www.nscale.com www.nscalesupply.com and
www.wig-wag-trains.com all of which I have found very helpful.
As for locos, I've found Kato, Atlas, Intermountain all very good. I'm sure that current Microtrains. Lifelike and Bachmann Spectrum are also okay.
Look out for couplings. Older items, and some manufacturers (e.g. Lifelike, Modelpower) use the non-prototype rapido couplers; most other current makers use the prototypical knuckle-coupler - look much better! The two types don't mix, so if you get rapidos you'll have to buy replacements if you want them to run with knuckle couplers (easy to get from various makers: Atlas, Microtrains).
Also - as has been mentioned - if you use Atlas Code 55 track, or Microengineering Code 55 or 40, you'll have trouble with cars/locos with wide flanges. Most makers these days fit, or supply narrow flanged wheels. You can get these easily and switch wheels or trucks without too much trouble.
Good luck with the project!