Depends on the number of routes. If you want three feeds with full routes you should go to 1.5GB RAM. If you’re just getting defaults and announcing a few blocks of your own, it’ll do fine with the stock RAM.
Probably, but it’ll be close (I don’t have anything running full BGP on RouterOS, but two full table peers on a Cisco 7206VXR shows a little less than 512MB used). And since memory is used for other purposes, too, you could very conceivably run into issues.
RAM is cheap. Downtime isn’t. I don’t see the point in trying to save yourself $100 tops and taking a risk.
I don’t know if they sell just RAM - if they do maybe consider ordering through them as you can rest assured you’ll get the right stuff (down to which vendor, make and model of RAM sticks work best).
It’s one slot, and if you put in 2GB sticks 1.5GB will be usable as that is maximum the current version of RouterOS can address.