Hi!...OK...let's see...first,I strongly recommend that you try very hard to build slightly larger than a 2X4.In such small space,you can't really build any interesting pattern other than a simple oval,and that is with the tracks running right on the edge of your benchwork with not much room for any sidings on the inside either.If you could stretch to let's say 30 or even 36 in. wide and possibly 5 or 6 feet in length,it would make a world in difference.
Kato Unitracks are very fine indeed and any loco and rolling stock will run on them but I don't believe that they'll be any cheaper than Atlas tracks.For a beginner,Atlas code 80 is a good option in my opinion.Reliable,easy to install and at a reasonable cost,you'd be able to run just anything on them that can run on 9 3/4 in. curves.Obviously,you'll have to stay away from most steamers other than the pretty small ones and be careful with your choice of six axle diesels as most will negociate these curves but will still look strange doing it.
Elevations?...yes...but very mild ones.Reasonable grades don't exceed 2%,meaning that a 2 in. rise will need 100 in. of track length to not exceed that percentage.Considering that it is what you'll have in total mainline length around your layout and that the train has to come down a similar slope on the other side of the hill,you can't go much higher than one inch.However,you could cheat a little since you will not be able to assemble long trains.
Running two trains is indeed possible but you'll need a second mainline outside the first one,and this one will have 11 in. radius curves,wich will bring your trackwork right on the edge of your 24 in. layout (I have doubts that Unitrack carry 9 3/4 and 11 in. radiuses).Then you'll have to have a second DC throttle and design what is called "block control" to drive your trains.For this purpose,I suggest you consider DCC,a basic starter set won't be much more expensive if at all than all the hardware you'd need for block control and would make your layout more enjoyable.
Will any brand of N scale locos and cars run on any brand of N scale track?On code 80 track...yes.On code 55 track...NO.Many will but some older models won't without replacing their wheel sets.
Whistle and smoke?Smoke is nothing new,but in rare models of steamers only.On the other hand,sound does exist in N scale but has still a few leaps to go.Right now,the only drop-in type decoders for N scale diesels is from MRC but they are plagued with unreliabily issues.I'd avoid them.
As for FireAce's question...I've been in N scale for close to ten years now and have never heard of or seen any grain cars that could open in this scale and I doubt we'll ever see this due to the extremely reduced size of the mechanics that would be required.It is possible I think,but what would the prices be for such items and where could one find scale grain to have these somewhat prototypically operating?