Trail ratings are pretty vaguely defined, so I'd take them with a grain of salt. Ultimately the best way to determine if and how you need to modify your truck is to start on easy trails and work your way up to the point where you start running into the limitations of your vehicle. You'd be amazed at how far a stock truck can go. I'm assuming you own a Toyota since you put this in the "FJ, Hilux, Tacoma, 4runner" forum, but it would be helpful to know what exact truck you own.
I don't think there is anyway to specify a generic build list for level 5 or 6 trails. Some things to consider...
Driver skill
An experienced driver can run a more difficult trail with a less prepared vehicle.
An inexperienced driver may have issues on an easy trail even if they are driving a very capable vehicle.
Stock vehicle specifications
A stock Wrangler Rubicon (and many other Jeeps) are "trail rated" and with a good driver can go through the Rubicon trail with minimal risk of damage.
Other vehicles may require extensive modification to have the same capability.
Willingness to incur body damage
If you are willing to damage your truck then you can run harder trails than you can if you want to avoid damage.
How to handle getting stuck
If you often go with friends who can help you get unstuck then you can run harder trails.
If you go by yourself and need to be able to self-extract, or not get stuck in the first place, then you need to run easier trails or build a more capable vehicle.
Trail ratings are not consistent
Here's an example rating system for Moab.
http://cruisemoab.com/trail-rating-system.asp
I've driven a truck on 32" all terrains on several of the 5/6/7 rated trails with no issue.
My recommendation is to keep your truck stock and only modify when (and if) you need to. Aftermarket modifications are expensive can often be the source of reliability issues. If you post some information about your truck, your driving experience, and the specific trails you are interested in then people can give you some more useful advice.
- Matt