As I recall, the front and sides of the sedan and hatchback are the same. The Rear of the sedan is longer, so the ST rear bumper wouldn't fit anyways. This style of focus has been out since 2012, so you could look at older models and scrounge salvage yards. Or search online for aftermarket, but I don't see too many people on this website changing their bumpers or side skirts too much, so expect that option to be pricey. Also, expand your search to Europe, like Britain, where the Focus is more popular. They do like to add spliters to the front and I think I've seen side spliters too for ST's. You may be able to find rear diffusers for the back bumper also, but once again I think that they are ST specific. Those may or may not work for you.
Wheels. I like the snowflake wheels. The 2013-14 wheels are bright silver. 15 to present are a darker grey. In 2014 the grey color was introduced as optional, so it's a cross over year. And aftermarket, you can get the snowflakes in black from Ford Racing. If you like black, I'd go that route. I see them for $190 each. Alternatively, you can strike a deal with someone here or hunt local craigslist market. For a good set of snowflakes expect to pay $400. I've seen good clean sets go for as little as $350 and as much as $800. With used wheels, you take a bit of risk, so buyer beware. I've been lucky so far.
Making it your own and being to young to afford performance? Been there and done that. My first "newish" car was a 93 mustang LX with a 2.3L 4 cyl. I couldn't afford the 5.0 and even if could have, the insurance payments would have killed me at 22 years old. The first thing I did was track down a set of Mustang GT turbine wheels, which were big 15x7's and mount some 215/55r15. Going from the stock 185/70r14's, I thought it was pretty amazing! At least until I realized that the heavier wheels and wider tires actually made me even slower, and that car was not fast.
As time went by and I mastered driving my car, I began to learn the down side of owning a base model. I did lots of research and cost assessments. Power upgrades, like adding turbo, were out of the question because neighter the engine, transmission or rear differential could take over 160 lbs of torque, and I already had 130 stock. Suspension/brakes was the same story. I had different wheel hubs, brake mounts, control arms, sway bars and spring rates. Although I loved that car, my ability out grew it and the only thing that the car had in common with the 5.0 model was the interior and the chassis. (And in hatchback form, that chassis was known for it's flex, so it wasn't the best choice.) Mechanically, upgrading any of the drivetrain, suspension or braking components rapidly lead me to the conclusion that selling the car and buying a 5.0 was the better investment. So I moved on.
Many of the folks on this website are serious performance enthusiasts, who will cut through the chase and recommend the grippiest tire, lightest wheels, biggest turbo, best tune, etc... and consider all other options substandard and a waste of time. If you are going to the track, this is great advice. But if you are just doing your own thing, don't be surprised by the bluntness of the internet. Net-etiquette is not the same as face-to-face conversation. I like that you are doing your own thing. If you want to keep the cost down on modding your car, be prepared to buy stuff second hand or explore salvage yards. I get what you are doing and it's a lot of fun. My last sports car was a 2004 Pontiac GTO. Great car. Fast, handled good and could cruise for hours in comfort. But I never loved that car like I loved my 93' 4 cyl. Mustang, even though the GTO was superior in every way.
Go do your own thing, because it's your experience with your car.
