They'd bring in the whole shebang.
So my rhetorical question to you would be: why would you not buy a system with a reduced radar footprint, advanced avionics, advanced weapons control systems and advanced networking? Even if it's only for defensive purposes.
Heck, even if it's for flying strictly over territory you control. Advanced radars cover hundreds of kilometers as do modern anti-air missiles. They can shoot you down without even going near their direct airspace. That's what Russia is doing right now, they're taking down Ukrainian aircraft at no risk to themselves launching a bunch of R37s from deep in their territory (50-100km) from Migs Ukraine can't reach or even see.