I own tons of mods and have learned real quick and simple this rule:
1. Buy what you need, not what you think looks cool.
2. Pay attention to who you buy from. It's easy to see what works and doesn't work, the really good mods all have tons positive feedback on them.
3. Free usually means Cheap and Cheap will get you one of those cars someone else paid $200 for and Uncle Sam is now paying people $4500 to take off their hands. While there are some really good free mods, most of them aren't worth the time it would take to download it, let alone install.
So, go with options 1 or 2, those are your best ones. Option 3 will get you definition #3