In a situation where you're looking to use multiple themes on the same site, you may like to consider a WordPress multisite setup, which you can read about
here. With this you can also share a user database so that users aren't needing to register/log in more than once across the installations.
This is the first time I've heard anyone state concern for the price of our themes being too low to be honest ;) If you are interested in specific themes, you can of course find all the information regarding included features on the relevant
theme information pages. I think the main consideration is whether or not the themes include the features you are looking for by default, and if not, then it would involve customization on your part. The only reason you'd potentially "need" to spend more money is if you're planning to hire a developer to implement changes which are simply not included by default, but in terms of what is already part of the theme, you can expect these features to work as advertised. If you're not sure about information in the marketing material then please feel free to ask and we can clarify it for you. Essentially what I'm saying though is that you won't need to make changes to have our themes run correctly, but if by "correctly" you actually mean that you expect the themes to include every feature you want them to have, then again, this comes down to whether or not the features you desire are already part of the theme. Thanks.