I personally find that torque is much easier to work with than high horsepower, ideally I'm looking for a broad torque band so I have smooth power delivery throughout the rev range. With high horsepower cars the power delivery is much sharper and makes the car much more tricky to keep on the limit, even more so if you're not experienced.
Short gearing will cause problems as you want to be using as much of the rev range to control the speed the rear wheels are spinning rather than keeping the car within a specific range. You can't really change gears while the tires are lit as it will unbalance the car and so you will find yourself bouncing off the limiter constantly and not accelerating out of the corners.
Generally I don't bother with a custom trans as the fixed ratio boxes are usually more than adequate for the task, I can't really think of any circumstance where I'd genuinely notice the difference a full custom setup could make. You might be able to wrangle a few extra points in a seasonal event by getting the wheels to spin slightly faster but it won't make a noticeable difference for the most part. The only circumstance I'd change it was if I was consistently topping out or bogging down on a long corner, then it would just be a click either way on the max speed.
To stop over rotation try adding a little bit more Toe in on the rear, just take it up 2 clicks at a time until you feel comfortable.
Experiment with the Diff, there are no fixed rules as to how you should do it, a lot of people default to the 5/60/5 or 60/60/60 as these represent locked diffs. 5/60/5 will give you full lock under acceleration but will allow sharp adjustments when you lift which can help to keep the car sliding, 60/60/60 is a welded diff and gives unrivalled consistency at the rear wheels but needs precise throttle control. I like to run a slightly more open diff that will lock when asked but give you more grip when you need it and better exit speed.
All comes down to personal taste and what suits the car, the Initial and Accel are all you need to concentrate on. Keep the Initial quite low and the Accel quite high, you can make big adjustments and feel the results easil so it doesn't take long to get setup, makes it easier to understand what the different settings affect as well so will help you with general tuning.
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And that's one car of the planned 5.
