an i7 can often be replaced with a Xeon
From a quick search, available Xeons seem to be either more expensive or inferior to the i7 that I'm looking at.

I'm not entirely sure what you mean by warranty as individual parts should have a warranty as well
A machine warranty is valuable for obvious reasons.

you can certainly afford the "risk" of just trying a GPU-upgrade first.
Time and effort are also part of the equation.

I also like to buy at smaller local shops/companies if available, I don't know the situation in the US but the store where I bought most of my current PC hardware is pretty great and they are very friendly and helpful if there is a problem.
No idea, but there's a certain stereotype here...