Welcome to SMF! You'll find the answers to all your questions here. The best way would be going to the charcoal smoker sub-forum (go to Forums) and scroll down till ya find it. Scroll through there for anything related to offset smokers. There's quite a variety of mods, some you'll choose to use, others not.
The biggest things to look at doing would be to use some dryer vent pipe (metal) to lower the chimney to near grate level; add a baffle in the cooking chamber to deflect heat/smoke further into the chamber; add tuning plates to further direct heat/smoke down the chamber; making a "box" out of expanded steel to hold charcoal in the firebox; and, use the minion method for the charcoal/lump coal. Also, mounting good quality thermometers at both ends of the smoking chamber will help you figure out the best combination of baffle/tuning plates. It sounds like a lot of work, and it can be. However, the results will bring you many, many compliments.
Not everything needs to be done at one time. For a budget, I would suggest the chimney extension (really cheap) and a makeshift baffle from a pizza pan or similar. The baffle might warp depending on temp, but mine is from 1/8" steel and works great.