Can trackpad be replaced with a touchpad? What's my max RAM capacity?

You can get up to the MASSIVE 8GB! Sadly, that’s it for RAM as the Core2Duo Penryn mobile CPU only has address lines to support 8 GB.

And sorry a square peg won’t fit a round hole either, Apple loves altering the design as newer and better tech comes out, and the physical size of the touchpad also prevents it.

Sounds like you need to upgrade your system to something newer. I would look for either a 2011 or 2012 13” or if you want a bigger screen stick with the 2012 15” All will support 16GB of RAM but they still have an older trackpad (glass vs mylar) otherwise your aiming for a 2014/15 retina system but be careful! This series has solder RAM so you can’t upgrade later.

Then to get better Force Touch trackpad you need to go with the 2015 model, The 2016 onward systems have the massive touchpad, but then you have everything soldered in and only have USB-C ports.