Well it depends on how much scope you want in your characters. There is the Ethnicity Bundle, various Merchant Resource Kits and individual Genesis characters all add to the morph gene pool, so any character will give an ability to dial in portions of that character into other Genesis characters. Individual Genesis characters can really expand your morph capabilities, especially in the fantasy realm, but also in ethnicity, age etc…
The main thing is that you want to stick with Genesis or newer (characters with a 5 in their name) generally. You can use Gen4 characters for their skins if they have a particularly nice one, mainly if they are fantasy skins etc… but beyond that Generation 4 and earlier characters will probably be too much work to hassle with in general for various reasons. Gen4 actually has a higher poly count while being less flexible, and Gen3 are usually too simplistic for many situations now. The one caveat to Gen4 characters (and occasionally Gen3) is that they sometimes will have a particular character in that generation that isn’t available in Genesis due to being around for a while and being a very mature market. There are gen4 characters that mimic rl people for instance that aren’t available in Genesis.
If you like the ‘look’ of a Gen3 character for toon type renders, you can get Gen3 Iconic Shapes and that will give the base Gen3 characters that can be mixed into the Genesis gene pool.
Gen 4 Iconic Shapes are generally used fitting clothing from Gen4 characters to Genesis so probably not something you would be interested in.