I have been using C1 for quite a while now. Balinor developed one but never made it a public mod.
Anyway after installing C1 our conversion rate improved dramtically. We went from around a 30% conversion rate to over 50% and the mod basically paid for itself the first month. Yes $169.00 is a big chunk for a small store, especially since the cost is more than X-cart. If you do not install it you should make sure you get Google Analytics installed and see where your customers drop out. If it is consistantly in the cart then you will need to improve the checkout process.
Yes C1 is not available for 4.3 yet. I am currently running 4.1.12 with C1 and I am customizing 4.3 in a beta directory and will upgrade the main site once Bill gets a 4.3 version of C1. To me it is worth the wait even though I want to have all the bells and whistles of 4.3.
So basically the most important mod would have to be C1.
Mike