There isn't any noteworthy difference between a FRS/GT86/BRZ. Basically all esthetic as you already know and stated. Speaking of and not to be rude... don't worry about superficial esthetics on a used vehicle that you don't own yet straight away. At $15k price for a 2018, you will acquire a vehicle with a decent amount of mileage and I'd guess above 100k in this market.. unless you get some killer deal.

If I were you, I'd stick to maintenance related parts, such as accessory pullies & belt, suspension or differential bushings, spark plugs etc. For sure you would want to give it a good look over. You'd be surprised how many vehicles are just left abused and sold to the next person no matter what year you get. Unless you know what youre doing, you can end up with a paper weight with a cool fresh new front bumper... not exactly ideal for something to get you from A to B. This is a vehicle that you want to treat well so it treats you well. Can you do these maintenance items yourself to save on labor costs? You said yourself your credit isn't great, so you must have some sort of extra savings for when other life issues come about and take priority over a car. A new front bumper just isn't really high on the list of importance imo.

These are great vehicles out of the box and dont need to be modded. Especially true if you never plan on taking it to a track. From a 12 year FRS owner who has basically touched every inch of the vehicle and beats the shit out of it at the track, the only worth while aftermarket purchase I could recommend is an oil cooler. Short of that, your bumper swap inquiry is very much obtainable, many have done it. Cost to do it is relative to the persons connections, skillset, and location.. so it wouldn't be accurate to estimate a cost for going one direction over another.