Need Suggestions for iMac Replacement

Hello Community, I would truly appreciate any suggestions and feedback you may have on our iMac replacement dilemma. We currently have an early (January 2006) 20” Intel Core Duo Mac, OSX 10.6.8 with a 250 GB HD that has served us well these last 9 years, however as more applications are written for more memory, 64 vs. 32 bits, videos and pics, the Mac has become a bit wonky and it’s becoming difficult to update iPhone and IPad info, so it’s time for replacement. I have reviewed the new iMacs and MacBooks and am unsure which system is best for us. Here’s our situation:

1. We want the system to work effectively as long as possible. Another 9 years would be nice. This will require more memory and drive storage in anticipation of bigger future technological changes. For example, storing movies on the hard drive wasn’t a “thing” when we bought our Mac. We are quickly filling our HD up with videos, music and pictures. Using iCloud for storage is not very feasible because of the following:
2. We live in the country and do/will not have access to high-speed internet through land-based cables. We recently cancelled HughesNet Satellite when AT&T upgraded the 4G LTE equipment on the closest cell tower to our home. Our only internet access is now through an AT&T hotspot. It has worked well with iPhone software updates and movie downloads, but I don’t know how well it will work with a very large OSX download.

Because of our internet access situation a MacBook with a docking station and monitor seems like a good way to go since OSX is no longer available on CDs and Apple allows owners to take their MacBooks into the store to download the OSX updates. The down side appears to be dual vs. quad core processors and memory size. I also have an iPad and iPhone, so I have no need to travel with a laptop. Cost is always a fact in these decisions.

The iMacs are wonderful, and have plenty of options. Do we need quad-core vs. dual core? And, what options do I have to upgrade the OSX software if AT&T becomes cantankerous?

Community, what are your thoughts on our situation? Which Mac system do we need?