Andy, just
Seriously - my $0.02 worth: If you are OK with a Diesel first, then maybe a plastic 2-rail loco such as Atlas SW, MP15DC, GP7-9-35. Those can be run on a NCE Powercab.
If you fall for a plastic or brass loco with a Pittman or similar large can motor then the ESU CabControl system with its 7A booster would be a better bet. The decoder for such motors will cost between 1.5 and 2x the HO one you could use in the Atlas locos. Examples of this type could be Weaver RS3, or GP38-2 which can be had cheap, but check they are complete and run well. Don't buy an older loco with an open frame motor unless you are up for a re-motoring project before you can run with DCC.
DON'T buy a DCC decoder until you have bought the loco. Then tell us what you bought and we can recommend the right deocder models you can choose from. 
Freight cars: Weaver and "Old Atlas" freight cars can be had cheaply but are lower detail. New Atlas freight cars are generally very nice and priced accordingly. MTH and Lionel have some nice scale-sized cars that can be converted easily to 2-rail, noting Matt's good point about the coupler pads.
You may need a few packs of Kadee couplers to convert with. And you may see Atlas, MTH and Lionel 2-rail conversion kits of just wheelsets or complete trucks for any conversions needed.
Track-wise, there's "old Atlas" which is a bit clunky but quite usable. That comes in flex sections and #6 (approx) LH and RH switches. Black ties and chunky details are the clue. Usually some used stuff at shows. It's from the 1970s and '80s so check the quality if used. Don't buy the oldest Atlas ~24" switches as they are too tight for the above locos.
New Atlas - brown ties and better detail - comes in flex sections and about 4 radii of sectional track. Plus #5 and #7.5 LH, RH and Y turnouts. More realistic and more expensive.
There are a couple of new makes of 2-rail and P:48 turnouts, Signature Switch Co and O Scale Turnouts. Both are excellent in slightly different ways and cost about the same as new Atlas.
My wallet is already whimpering in sympathy with yours - have fun!!
