hey lady...this is going to be a long-winded answer! Great question by the way! It is a very fine line for dirtbikes between getting one you can control and getting one that you will have to "grow into". Usually parents try to get their kids bikes that they will need to "grow into" to save some money and it usually is a huge disaster as the kids are less confident and don't have nearly as much fun.
Sounds like you are an adult so that really isn't a huge concern for you, but at the same time, confidence is key and as cool as it is that you can ride horses and quads, dirt bikes are still a different story. It is a great idea for you to buy new as you do not have to worry about some idiot's terrible maintenance schedule!
I am also 5'7" and about average for size and weight and I ride a Yamaha YZ 125cc two stroke motorcycle.
OH, I digress - you asked for some help with "numbers"
Okay- this is how it works...The number that describes the bike denotes the engine size. After a certain point the number does not have much to do with the physical size of a bike. Basically after 125 - the bikes are almost the same in height - but very importantly for you specifically - not the same in weight. There are two types of engines used in motocross/dirt bikes: 2-stroke and 4-stroke. They difference between these two is in the engine. It has to do with the amount of time the piston moves up and down with the combustion of gas/spar/air. A 2-stroke's piston moves up/down for one "burst" of combustion. The unused fuel and oil basically burn off and exit the muffler (silencer). A 4-stroke is just that: up/down/up/down. What that means as 4 strokes produce less oil/gas filled exhaust which is much cleaner for the environment. Because a 4-stroke has twice the "strokes" - they consider the diplacement to be half. IE: a 125cc 2-stroke is equal to a 250cc 4-stroke. and so on...
There is a noise difference between these two types of engines as well. 2-strokes are higher pitched and the sound is in the arena of a chain-saw or other 2-stroke appliances. sorry- a chainsaw is not the best comparison, but it might help give you an idea. A 4-stroke is much lower sounding - kind of lthunderous.
4-strokes are louder on a decible scale but a little less piercing to the ear.
Another major difference between these bikes is maintenance. Because 2-strokes are much simpler engines, they are relatively easy to work on - even for a beginner. Besides having their oil changed ever 10-20 hours of riding, they need "top-end" work periodically - depending on the amount of use. "Top end" means the piston, piston rings and bearing need to be replaced about twice a year if you are riding pretty steady. This is pretty easy to do with the right tools and a manual. Also it is really not very expensive. The worst case scenario is that the engine seizes and you have to either replace you top or bottom end- which in a 2-stroke is between $600-1500 depending on the damage.
This is actually relatively cheap compared to the repair of a 4-stroke which can be $2000-$4000 to fix an engine. Because 4-strokes have much more complicated engines - they are not so beginner friendly. The valves need shimming a few times a year and also they need seriously frequent oil changes including new oil filters everytime-unless you get a metal mesh reusable filter.
Like I said earlier - I ride a 2-stroke and I'll tell you the reasons why - this might help you the most.
1. It is lighter than a 4-stroke
2. It is cheaper than a 4-stroke
3. I can fix it ALL BY MYSELF!
4. I can start it if I crash. (four strokes are notorious for being really hard to start if you stall or crash in a race-even the new ones that are better than the old are hard to start-especially for a girl who is just tall-enough, who is tired form racing, then crashing, then picking her bike up out of the mud she probably just crashed in)
4-strokes are a much more forgiving bike if you are in the wrong gear, the power is more available quicker - therefore they are easier to jump and better at starts for some people.
Whether you go 2 or 4-stroke, I think that a 125/250F (F stonds for 4-stroke) is going to be more than enough bike for you. If you want to ride MX, I would not get a KDX! That is almost stricly a trail bike. Motocross bikes have stiffer suspension design specifically for tracks and racing!
Like I said - I ride a YZ 125 and I really like it. The power is very smooth. Suzuki RM's are really nice to ride and I find they really corner well. The powerband is very noticable compared to the YZ! - but it is also helpful to know when you are in the power-band (the spot when you are accelerating where your bike has the most pickup in that gear.)
Good Luck in MX and I hope that this helped!