Here's some info for you:
1. If you have all the points books and old WW stuff, you don't necessarily need a new Momentum kit. I've done the old Flex/Core and am doing the Momentum and the Momentum is a mix of the two. You now account for ALL foods using the old points system, but try to maximize "filling foods" (CORE foods) within your day to avoid being hungry. I still use my Flex food points guide from 5 years ago, so don't worry about it.
2. Here is how to calculate your points according to the new Momentum guide. This is available online, and is contained in this thread:
How many weight watchers points do I get daily?
3. The last "difference" is that you get a set number of points per day (vs a range, as per the old Flex plan), and 35 points per week, plus exercise points. Everything else is the same.
It sounds to me that you may want to give it a shot using this forum for support, and spend the $40 on the gym and I think that is perfectly valid! So go for it!
I hope this helps, and feel free to pm me if you need anything...