i was wondering if i just cut my exhaust before the secondary cats will it trip on my check engine light?
yes
if so how would i fix this?
You need what is called a mil eliminator. It simulates the signal that the downstream O2 sensors send to the ECU. It will not affect how the engine runs, since they just are for checking catalyst efficiency.
and also, where can i get a brush guard for a reasonable price?
ebay, summit racing, lots of places. Depends on the budget you have and the quality you want. If you just want one for looks, they are on e-bay for a couple hundred bucks. If you want a functional one, Westins are getting harder to find, as are the other quality brands, but they are around 400-450
im in high school, just joined the airforce and have a job working on a wheat straw farm for 8 an hour so i dont have much of a income at the moment.
Congrats on joining
i want to do the 3 inch body, but will it look stupid with stock tires?
Well, this question is entirely subjective, but yes. Also, why are you looking at doing a body lift? Do you just want the appearance of a lifted truck, or do you want some type of functionality?
and i also have around 227k miles on her, will it make her die faster?
not really, but you may want to consider devoting some of your limited funds to maintenance and future repairs.
i want to put led strip lights on the underside of my mirrors, but cant figure out how i would do so. there are lights there already, buy ive taken it apart and still cant think of a way that dosent look like crap honestly. any help is appreciated guys!

It is already equipped with puddle lamps? Are you looking for more light output or whiter light? Instead of using strips, search on here for "Puddle Lamp LED" and you can find numerous options for brighter, whiter puddle lamps. You can also use e-bay plug and play LEDs. Stuff like led panels work good for this kind of thing, but measure carefully before you buy. The best way to go is to make your own. It isn't that hard to do, and even beginners can make decent stuff if they take their time and aren't scared to try it out. If you need help with that kind of project, there are several of us on here who have done that kind of thing, and you can look in the performance lighting section.
However, another option you may wish to consider is to mount those strips to the bottom of your truck's body near the door, and still wire them to come on with the courtesy lamps. The door blocks a bit of the light the puddle lamps make, making the puddle lamps much less useful than they could be when you are stepping out of the vehicle.
Lastly, the Grammar Nazi in me would never forgive me if I did not point out that, since internet forums are a medium of written communication, it is proper etiquette to capitalize the first word of sentences and pronouns properly. Writing properly makes you appear more intelligent, and makes it easier to read what you have written. Furthermore, it is good practice for IRL communication.