honestly here's your easy solution for aux input
maybe not the cleanest but for $10-$20 it's taken care of. spend the rest on speakers if you want an upgrade (although lots of people will say a better head unit is the first step to better sound).
there are ways to add an actual aux input to stock headunits by soldering aux wires to the boards in the unit and can then be rigged to play with tape active (or possibly cd changer) but you'd probably have to get on a toyota forum somewhere for more information. i plan on doing this soon for my nissan but ski season eats up my weekends and i havent really had a chance for a few months.
as far as making a new HU/speakers a DIY job- i was able to figure out how to install my own shit at the age of 16 so don't think it's something too hard to handle. just depends on how you value your time/money. at 16 $60-$80 was a lot to me, now maybe not so much so i don't have to fuck with it.