There have been many discussions on this forum about how to make a round column mill do precision work. You have what appears to be a very capable machine, if you're having trouble getting the results you want from it I would suggest trying to resolve them with your current setup before re-designing it from the ground up. It's very possible you could make the changes you're talking about and still not get the desired results. Sometimes the only solution is a larger, more rigid, more precise machine to do the job, and sometimes you can get it done by modifying the process and/or materials used.
The rail you have might be good for a surface grinder or CNC router, making it work for a vertical column would be challenging to say the least.
Post up some pictures of your shop and the projects you've worked on, or at least tell us what you're planning to do that you don't think the G0791 won't handle. It's easier to figure out how to make one part than building an entire machine, ask me how I know this....
As for storing your mom's fridge I can understand that. I recently moved a bunch of my mom's stuff into a rented storage unit so I could expand my shop. For me it's worth the extra monthly expense to be able to use my garage as a shop rather than storage for stuff that will likely never be used.
Cheers,
John